Are infidelities really that common?

My understanding is that infidelity is very nearly binary in its commonality.
There are groups of people for whom infidelity is normal, it is the norm. They believe that everyone cheats, and in their experience everyone does, because they are cheaters and are friends with cheaters. They believe that fidelity is impossible, and claims to the contrary is just social posturing
Then you have groups of people for whom infidelity is basically unthinkable. That it is the greatest breach of trust possible. It is not just not normal, it is non-existent— you don’t cheat, your partner doesn’t cheat, your friends don’t cheat, no one you know cheats. If someone you know cheats, or someone known by someone you know cheats, it is legitimately horrifying: this is not merely social posturing, it is literally shocking to you, because in your world, cheating simply does not happen. It is horrible.
Cheaters think everyone cheats. Non-cheaters believe no one cheats, or only horrible people cheat. These two groups tend to self sort themselves into groups. Bad things happen when the two groups intermingle, in fact.
What’s also a tragedy is when someone who would naturally be in the non-cheating group ends up, mistakenly, in a cheating group. They will begin to feel like everyone ELSE in the world cheats, while they themselves never would. They keep getting hurt, they keep getting betrayed, and they don’t understand why. They need a better friend group… and let me be clear: non-cheating groups ABSOLUTELY EXIST. Those groups simply don’t interact with cheating groups— they basically don’t even know that the cheating groups exist, and would be horrified to find out. So if you’re caught up in a cheating social circle, getting out is really hard! You need to find people who have literally nothing in common with the people you already know!
It kinda sucks. I don’t know a solution.
- Huh. I don't think I know anyone who cheats so I guess I got lucky. Your post is plausible
  
  From time to time I get a ride from someone at work I have zero interest of becoming friends with. In those rides I get glimpses of a complete different reality where he and his friend group lives. It is horrifying and it completely matches the description of the parent comment.
- Yeah. IDK how people can fit cheating in to their life.
  I just don't have that much time apart from my spouse I guess.
  Also I'm not particularly attractive I guess so even before I was a balding 40 something I've never encountered someone who wanted a quick fuck in the broom closet or whatever.
  I'd like to believe I'm a non-cheater, but what I'm saying is that's never really been tested.
  Also having kids changed things. I wouldn't want to betray my partner, and that's very important to me, but it's a lot more important to me to not mess up our young family. My spouse and I are romantic partners, but we're also a parenting team.
- Absolutely. I belong to a non cheating group. It's just seems completely unfathomable that it could happen. Most of us are in 15+ year relationships and are friends with everyone. It's not just a "the women are friends with the women, the men are friends with the men" situation. We got a blend of genders all participating in the same hobbies. There would be so much social cost to cheating it would be kind of insane.
  Where I work though there's a decent amount of drama in that regard though and I have noticed that one common factor is that the relationships are atomized. They either keep their old friends going in and there's almost zero expectation of their partners integrating into each other's friendships or there's just this expectation that men and women are fundamentally different creatures. That whole men are from Mars women from Venus shtick. From the outside it seems like emotional distance where people look at each other like they aren't targets of empathy - more like they play by a book as if they can just put the right inputs in they will get the desired outputs.
  I know this is entirely anedotal and that anybody could theoretically cheat for any number of reasons... It's just something that I noticed about the groups of cheats that I am aware of.
- I don't know anyone who cheats... Except my Uncle Chris, who has been unilaterally excommunicated from the whole family.

I am not one to force monogamy on people, though I personally am not interested in being with anyone but my partner. That being said, I think it's wrong for anyone to violate a partner's trust. I find it messed up that your high school peer bragged about betraying someone who she claimed to care about (at the time). In a nutshell, it seems like it would be good for society if we got over our puritanical zeal for enforcing monogamy, since some people clearly are wired to need multiple physical partners while still wanting to emotionally connect to one person. But I personally would continue to only be with my chosen partner.

I guess this means that cheaters are more inclined to go to school reunions.
- Seriously, I think that fits. Many people (not all) cheat because they are unfulfilled in life. And the same cohort also might go to class reunions for the same reason... to try to find fulfillment.

Infidelity is somewhat common but I would say it’s not “normal” at all to openly discuss and laugh about it at dinner with a bunch of people that you haven’t seen for years.
Seriously- wtf is wrong with these people? That one person sounds especially horrible.
Also - don’t go to any more reunions. I’ve managed to avoid 40 years of that shit and I like to believe that I’m happier for it!
- I went to my 10 year to make sure I was right. I was. I haven't gone to any others and don't even get asked.
- Yes, Right? It was too much to talk about it. Definitely that was my first and last reunion
- The only thing I get from meeting again people I haven't seen for decades is to, using the abilities I've been acquiring with time and life experience to read other people beyond the superficial, find out that ~~most~~ many haven't really mature much from the people I knew and at times how much I misjudged them back in the old days when I was very naive and ran around pretty lost.
  The "I'm better than that" feeling would be highly satisfying if I was a different kind of person, but it's actually just sad that some people turn out to either having always been less than I made them up to be in my mind or failed to actually turn into well balanced mature adults.
  The other possibility is that it's all in my mind and I'm just deceiving myself, as having become more more self-deluded when it comes to others with time looks exactly the same from the inside as having become a little wiser in interpreting others.

IIRC it's like a half or a third of the population that cheats, when research has been conducted. So, it's normal in the sense of common. But, like others have said, your reunion people sound like were trying to convince themselves it's normal in the sense of acceptable. And anecdotally it does tend to be the same people over and over again.
Polyamory is also getting mentioned, but that's a different thing, and poly people are a couple percent of the population at most with far fewer actually living the lifestyle.
- IIRC it's like a half or a third of the population that cheats,
  That's always what I have estimated the percentage of assholes or bad people from personal sampling throughout my life.
  
  I'd actually go way higher. The ones that seem nice are the easiest to externally pressure into doing bad things, which counts as being a bad person.

No, people who commonly do bad things, will often justify it to themselves and others by thinking most people do the same thing. And despite their protesting and arguing, they're pretty much always wrong.
I've heard more than one petty thief confidently claim that "everyone" steals something like a candybar when they go to the store. And it's why there are so many stories of cheaters accusing their partner of cheating.
- In summary: Projection.
  The chosen form of most accusations can be explained by the one doing the accusing having reached their own conclusions about the likely actions and motivations of others by thinking "What would I do in their place?".

The "infidelities are always there" part sounds like a cope. I'm not a great authority on relationships or marriage as I've mostly opted for single life but I have spent a lot of time around the various aspects of substance abuse and the way you describe this discussion sounds eerily similar to people with substance-use issues saying "everybody drinks" or "everybody uses drug x." It might be true of the people they surround themselves with but it's not representative of the broader population as a whole...
-On a seperate note you're really making me want to attend my 20th reunion next year. /s
-Lastly I just want to clarify that i'm interpreting the term "cheating" to being different than a poly/open-relationship. As I understand there's an element of deception or lack of communication when it comes to "cheating."

Cheating is complicated. Yeah some people just will, they are fooling themselves when they say they won't. Some people find it sexy, they literally want to cheat, they think it's hot. Others will if the situation gets extreme, people in dead bedroom situations who want to keep their family together and make the calculation (or miscalculation) that cheating is less damaging than divorcing. I worked with old people when I was younger, my bosses were old Spanish people who married for business reasons but had lovers, they did not love each other in a romantic way.
I'd say that as divorce becomes easier to get, marriage more based on love rather than alliance and monogamy less required, there is probably less cheating. But it won't ever be zero.
If you are asking is cheating universal? No. It's not.

I think it would be less common if people accepted that monogamy is not the norm. Just be honest with yourself and your partner(s). If you want a monogamous relationship - commit to it. If not - don't start one. I found out that I am polyamorous the hard way by thinking I am some kind of monster because I couldn't just be with one partner. I did not cheat but kind of always had other love interests. Fortunately, my partner of 20+ years now was able to accept this as part of me. And now there is no risk of cheating for either of us because we can just be honest if there is someone else. It doesn't change my love for him, it even made our bond stronger because it needs even more loyalty and acceptance of the needs of your partner to let them be.

I can't answer as to frequency, but I can say that recently a woman I know slightly cheated on her husband of 20 plus years, and the reason I know this is because he hired a private investigator and put the video of her kissing the other dude on Facebook. Apparently he did not include the video of them having sex but it exists. That seems like a really heinous thing for him to do, but if you know this family they're actually really well raised good members of the community, and he's clearly extremely hurt, stating that the depth of her lies to him was unreal. This woman's mother is a saint walking upon the earth, without a shadow of a doubt, and her daughter was what seemed to be a very ethical well brought up mother herself raised in the Christian faith (not the conservative kind but the really decent person kind), so this was all a big shock. Apparently she was just lying to everyone, including her saint of a mother, all along. Her husband asked her to stop the affair (which was with some guy she had known for years) until they separated houses, but she kept on and kept on lying about it until she was caught on tape.
So until someone's mask slips, you just really never know what someone will do.

In my experience yes.
My ex was always rambling about how men were always unfaithful, "oops I slipped and my dick ended up inside her" she used to say, to describe how easily men are unfaithful. She made it absolutely clear she wanted a monogamous relationship.
I was never unfaithful to her, but of course it turned out she was herself unfaithful numerous times, and it was crazy how bad she was at hiding it, almost like she wanted me to break up. So I did, and good riddance!!!
Later when I was moving in with a new girlfriend, and I was collecting some of my things at my ex, she was all dressed up, and all over me, kissing me and trying to win me back. She tried to kiss me on my mouth but I turned my head so it was on the cheek.
My girlfriend was waiting in the car outside, and she saw the lipstick on my cheek when I came back, obviously not too happy about it. But I explained it was all my ex and not me.
Lucky for me she didn't ditch me, and later my new girlfriend agreed to become my wife, and we've been together for 20 years now! 🥰
A friend of mine had just bought a very expensive apartment in Copenhagen together with his girlfriend. The papers were signed and the deal was closed. There are a few days where you can get out of the deal, and in that period my friend was told his girlfriend had been unfaithful. He didn't believe it at first, but the day after the deadline for getting out of the purchase, his girlfriend broke up!
I'm not saying women are worse than men, but the idea that men are more unfaithful than women is bullshit. There are more men than women in the relevant age groups, so obviously on average women have more sex than men do. That's simply a statistical necessity.
- Hah I've never heard the Pigeon Hole Principle applied to infidelity. Pecker Hole Principle?
  
  Pigeon Hole Principle
  Thanks I never heard of that, but that seems to perfectly describe the principle.
  
  I never heard of this Principle, despite having applied this myself to male/female dating society (including infidelity).
  I always imagined it as a Pairing Musical Chairs Principle, where m and n are the amount of chairs and people in team A, and n and m in team B, where n > m.
  Thus team A mostly rushes for any chair doing little else, while team B either don't bother playing the same game and instead decorate their respective chairs at team A or they fight for the taken chair that's always greener than the free ones.

Cheating is like divorce it's common if you look at the percentage of relationships. They are both less common if you look at at individuals. Cheaters always cheat again, those who get divorced often do so again.

Cheating is definitely in the minority of my social circle. I know of 2 amongst 100. At least people who were shameless enough to be admitting it.
That said, I do have maybe 5 open relationships in my friend group? I don't usually ask, so maybe 10 total. My wife and I are monogamous. If you're not, whatever, you do you. It isn't a religious thing, it's just how we roll.
But good lord at least talk about it, agree on it and stick with it. If you don't then you're a dick.

Married 20 years, never thought abut cheating, got divorced , and remarried 15 years now, and never think about cheating. If you commit to somebody, they should be your one and only, IMO.

For some people it is pretty common. For others, it isn't. The thing is: the people that treat it as a common everyday thing expect that to be the case for everyone - so if you interact with them routinely, you'd probably think that everybody is out to cheat at all times, but that's definitely not the norm.

If you are really interested in learning more about infidelity I would suggest you read / listen to Dan Savages's columns / podcast. Unsurprisingly there are a lot of different calls and discussions about fidelity and monogamy. I would also suggest you read Ester Perel groundbreaking novel The State of Affairs: Rethinking Infidelity
To answer your question directly infidelities are fairly common because monogamy is difficult and society typically puts a lot of stress and pressure on monogamy that makes it even more difficult. The foremost is the idea that monogamy as a default setting and that one person can be everything for one person sexually, emotionally etc.. So we have a situation where two people are assuming that their partner will be the only person they can be attracted to, the one person that can fulfill all their emotional needs and will have to be a perfect roommate / life mate. This coupled with the idea that you have to be perfect at monogamy or you are a complete failure at it. So you have a hard situation and hard expectations so people slip and some people who slip on something small (a micro-cheating which is a ridiculous concept) they go all the way. All these expectations are common in "Straight-land" while those in "queer-land" have a different set of expectation that work better for everyone.
We should all make monogamy an regular opt-in conversation for relationships (I would suggest ~6 months in when you go exclusive and then at most every 5 years). Moreover, we should understand our monogamist partner finds other people attractive and chooses to not pursue anything not that they don't find anyone else attractive. Finally, we should understand a our partner needs friends, hobbies and confidant who are not us to rely on. Until that happens "cheating" will likely remain something that happens fairly regularity
- That’s interesting. I wonder how much religion plays into it. My marriage is monogamous but we are both open to each other about attractions outside the marriage. It is OK to be turned on by someone else or some other situation, and we can use that together. It also helps that we both don’t generally like other people; we have a close knit group of friends but most folks we meet outside that are obnoxious and gross.
  
  Like everything there's some puritan morals involved. Most of which are around how sex and sexual desire are gross and not important. Couple that with the idea that suffering makes you a good person they all connect.
  Also I think media plays a big role as well. Some toxic relationships are glorified in rom-coms and other media. Add in the normalization of weaponized jealousy in reality TV. Throw in the new micro cheating that the Internet and relationships experts who make mountains out of molehills. It's a complex mix

Cheating makes stories, gets talked about by both sides and is overall contentious. Fidelity is literally having nothing to tell, so when a cheater talks, it seems like everyone is cheating, but that is far from the truth.
- Best answer.
  A lot of people cheat at some point in their lives, but most have the good sense to be less flippant about it. People who act like this are not the kind of people that you would want a relationship with anyway. You're not rare, but they're not common either.

I've seen a lot of cheating. So I suppose it's common. Not the norm, but common.
In the end it all just boils down to people not being really good on general.

I personally don’t cheat, wouldn’t cheat, being Demi/ace. But unfortunately everyone around me either cheats or has an open relationship then gets jealous when their SO gets some action. A lot of them have STDs.

Tw: child rape, abuse, drugs, cults, alcoholism, all kinds of random stuff.
Nah that person's a piece of shit. Everybody laughing could be in a state of internal WTF JUST LAUGH ALONG or actually be part of whatever shitty culture. A lot of places legitimately don't see how this is just insanely bad emotional maturity.
Remember though, people are VERY dynamic... Whenever you think you know the range of what people are like, just remember that you're wrong and you ain't seen nothing yet, and there are still far more conservative and far more liberal, and far smarter, and far dumber, and far more messed up stuff out there.
I dated a dude last summer that was telling me about a trans girl he dated years ago that grew up in a trailer park, hooked on meth, and was habitually raped by adults her whole life and everybody in that meth cult thought it was normal.
I've known wealthy people who were good people, wealthy people who were bad people, poor people who grew up and embraced greed and became rich, slum lords, teachers that cheat on each other and end up trading spouses and remarry. Old retired people having orgies, bdsm people who want to be whipped bloody to get off, and also 40 year old virgins and people who will never, ever find anybody because of circumstances and they think it's okay. Forgiving people, people who hold grudges, people who judge blindly, people who live in their own world of privilege, people who are miserable from listening to nothing but npr, sociopath guys who want to get with me, girls who flirt with me but rob me, people absolutely addicted to drugz nonstop everyday but otherwise are perfectly functional members of society, people with iq of 50, people with iq of 170. Families that behave like Mafia and families that behave like community leaders. Families that barely resemble families
There is always a bigger fish in the pond. There is always somebody who pushes your understanding of what an npc can be. Everybody's so incredibly different, yet in a certain light, we are all the same.
My point is, be yourself. If it sounds off to you, then it isn't a good fit for you. Use your best judgement, try to think deeply to define your morality by reading some simple philosophy - or not. You're special and unique and that's okay. Life is a long journey of finding what you like and what likes you back. It really is worth standing up for what you believe in, but also try to keep an open mind and at least truly understand what other people's lives are like. But you still need to put your own mask on before you can help others. And remember, it's your life, it truly is what you make of it.
People are all dumb though and make mistakes. We're just animals, all learning by making our own mistakes. Some not learning, some deciding that they want power, others deciding on bizarre motives.
You do you, and pave your own way. Be honest with yourself though, that'll save you a lifetime of time.

It seems fairly common. What seems to happen a lot is people get dissatisfied in their current relationship but don't want the drama or risk of breaking up.
For some odd reason it seems easier to meet people when you are already in a relationship so they take the opportunity to cheat while still staying with their stable partner.
- There's been a decent amount of discussion online that when you are already in a relationship, you've been "vetted" so to speak. Obviously bullshit, as what your current partner may find acceptable can have jack to do with what anyone else finds acceptable. Also, if you have a break up, somehow you're no longer vetted because clearly you have some problems if you had a break up.
  It's all pretty/petty bullshit but it's definitely a thing. Even my clueless ass has noticed it. Women are more comfortable around me now that I'm obviously taken (wedding ring). Not that they weren't before, but diving into deep/private conversation stuff at a far faster rate. Like I pride myself on being trustworthy and a good listener, but you gotta chill.
  That increased comfort also includes more comfortable to make passes at me. No thanks, wife and I aren't looking for a third.
  
  People are ridiculous. We truly are just psychotic hairless apes.

I think the quote was "that's what high school was, cheating and bad lunches". Or something like that anyway. I never cheated. I don't think I ever got cheated on either. I think in junior and high school a lot of people are figuring themselves out and make a lot of mistakes. But I think if someone is bragging about it as an adult, that's just shitty.

I think infidelity varies, but one thing doesn't: people who cheat and have zero remorse are not trustworthy.
You may have had a crush, but it sounds like you got lucky by missing out.
- I guess there is some cultural nuance - my impression is that for some people, sexual exclusivity is understood as an impossible virtue which it is important to appear to uphold, but where breaking it is kind of like sneaking a cigarette after having quit.
  Which doesn't make them untrustworthy necessarily, they just have a different understanding of how big of a deal it is.

You have to consider a few things here. You're not the only one with social anxiety at that event. Your reaction is not to go in the first place (my MO as well) or to sit quietly hoping it shall pass with haste. Others talk too much. School reunions are such a rich vein for neurosis because you're guaranteed to be judged by your peers. Peers who knew you very well when you weren't a more well put together person yet. Few people behave like themselves there. So if the woman says she cheated a million times lol, her neurons may be on the fritz as well because she's more thinking about how she dunked Sharon's head in the toilet in freshman year or whatever. And that memory is haunting. And she's sitting just over there! WHY HASN'T SHE SAID ANYTHING? ... So you need to have a salt shaker handy for anything you hear.
Also, some people like attention and will say anything to get it. People like to construct a public persona around their worst character traits, the ones they're unable to change. It's like they're putting a cool leather jacket on, aviator shades too. To distract themselves from their inner monolog, which very well might be telling them what a piece of shit they are.
And cheating is common. In my social group I know of a handful of cases. Drunk and horny, sober and crushing - the motivations are on a scale. In some relationships these secrets never get revealed, in others they've made the bond stronger, others have broken up. I would say very few people brag about it but hey, we contain multitudes. Some people end up in an unhealthy game of hurt oneup(wo)manship. Relationships are hard fucking work.
It is also a different picture when you have children with your partner. The willingness to forgive infractions increases for the good of the children.
And while centuries of indoctrination of monogamy and loyalty to your spouse can make this hard to accept: some people make open relationships work. I think it's more often than not the last stepping stone to disaster but if you can make it work, vaya con dios. I have a hard time with it but I'm trying not to judge.
None of this needs to change how you feel about cheating though.

I'm not sure what kind of infidelity happens in high school, but I suspect if someone cheated on me by kissing someone else while we were in high school, I wouldn't be upset, mostly because the nature of relationships in high school I don't think of as long lasting. I've never cheated nor been cheated on though.
- I agree. In high school is different. We all fall in love with a different person every day. But I think she was talking about her mid twenties.

I don't think I know anyone who's cheated. But I also hang out with a lot of polyamorous people, where cheating is possible but I feel like is less common.

All said and done, the concept that people should be monogamous isn't natural. Look at the animal kingdom. It's pretty rare.
Some people just don't fit that mold. But society tends to look down on them. So for some, bucking the system is probably a source of joy and achievement that they want to share. For the listeners, humans often go along with things to avoid awkward interactions. Then go home later and say to someone, "I can't believe she just said that."
- It's not the standard approach, but there's definitely examples. The trick being that swans have secret affairs fairly frequently as well.
  That being said, it sounds like you're talking about being poly, and as far as I can tell most philanderers just aren't. They cheat but don't want to be cheated on.
  
  Not really poly, just more fluid about changing relationships. Basically, more short term, not life long, commitments. When it isn't enough for a person, move on. And that shouldn't be considered a failing of the other. People change, interests change, newness wears off. That's natural. Society considering unmarried men as unstable, and unmarried women as somehow broken is just not right.
  If there are kids involved it gets more complicated of course. Then it gets into some kind of marriage contract or something. But raising kids these days is broken too. You almost need a professional involved full time just to keep up with all the things kids really need. Most of us parents just can't do it all, both from a time/energy perspective and a skill perspective.

The thought has never crossed my mind, and it would have never crossed the minds of anyone I know. I have no urge to cheat and neither does my partner. I have generally thought that it's a very unusual and uncommon phenomenon for someone to cheat, and I find it surprising how many people here think otherwise

I think monogamy might just be the uncommon thing, personally.

Welcome to The Against Matrimony Club, we have mimosas.
- The against monogamy club has coke and group sex

I’ve been on both sides of it. While not super regular, I’d say it’s not uncommon. Discussing it openly isn’t.
My buddy went to Amsterdam and had the restraint to only get handjobs and has never otherwise gone behind his wife’s back. I’d be surprised if any of my other friends did anything. One couple has guests over for mutual threesomes, so no need for infidelity for them. I know my ex hooked up with another female despite knowing she had a bf. People are ok with lowering their standards when an opportunity presents itself.

Incredibly common. I haven't seen any recent estimates but I recall at one time that ~15% of children did not have the biological fathers than they thought they did. It's not to point the finger at women - rates of infidelity may be even higher among men, it's just harder to track independently.
- I believed it was ~15% of fathers who did paternity tests, thus already suspecting infidelity, not overall numbers?
  
  This wasn't a published study. This was beers with the folks in the genetics lab at the hospital I worked at. No patient names or other identifying information was involved. They did tissue matching etc. and ran into the issue all the time. On a personal note two close friends have found out they had different biological dads than they thought they did. So maybe my perspective is skewed somewhat.
- It’s not to point the finger at women
  Just point it at humans unless you're talking about cheaters who step out to swing for the other team.

Some people are just shittier than others. What they look like on the outside has no bearing on their inner beauty.

Monogamy is socially imposed, not a biological prerogative. Therefore yes, cheating is common because social norms can only do so much to override basic human nature.
The natural human relation is a long term partnership without sexual exclusivity, which is common in other primates as well. This is why people cheat but still want to stay with their partners.
- If you don't want to be monogamous, don't, just be polyamorous and date other polyamorous people. It's a really bad excuse for cheating when there's plenty of relationship arrangements where this isn't a problem. There's no need to deceive unwilling people and cheat on them when you can find partners who think the same as you and you don't need to cheat on in the first place. You're still dealing with other people with feelings on the end.
  I'd have to really go out of my way to cheat on my wife when the only rule is to have safe sex (or be safe in general).
  
  This. But even in polyamorous relationships there can be cheating, which is simply going against the wishes and rules you set when going into the relationship. While some poly relationships are more open regarding casual or committed sexual relations with other people, some aren't. Cheating in every regard is bad, doesn't matter the relationship type.
  
  Wait. I never said it was ok. I was just stating the facts, I’ve never cheated myself and have always been in monogamous relationships.
- Found the cheater
  
  I have literally never cheated and have a,ways been in monogamous relationships . But those are the facts.