In United States, what are the benefits of getting legally married?

The right to make medical decisions on behalf of the other
The right to visit the other in the hospital
The right to make funeral arrangements for the other
The right to survivor's benefits (veteran's benefits, Social Security, private pension, etc.)
Income tax breaks and credits
Tax breaks on inheritance and estate taxes
Tax breaks on money and property transfers between spouses
Immigration and naturalization rights
Can't be forced to testify against the other (usually)
Communications between married partners are privileged from discovery in civil and criminal cases (usually)
Joint adoption rights
Bereavement leave
Joint bankruptcy protection
Automatic recognition of the relationship by every state, nation, etc.

Etc. There's something like 1,000 rights, privileges, and responsibilities that attach through marriage only.

Some of these make me sad to read because it demonstrates what at least my society lacks and what it has but uses as gimmicks, as if marriage is a cult.

I've seen married people forced to testify against each other all the time. The "right to remain silent" is universally thrown out the window.

The "joint adoption rights" thing is flipped around. If someone is jailed, the rights to their children is taken from both them and their spouse.

Imagine for a moment we even have inheritance taxes and rules against making arrangements for anything.

The automatic recognition of the relationship by every state and nation is a misconception. That's how it's supposed to work due to the Hague convention, and in regular cases it does, but it's not enforced.
It's very interesting to me that people tend to mention tax breaks for marriage because it's just not the case except if one of the spouses does not work or makes significantly less than the other. For example, when I was married, our taxes went up by about 6k a year. It's called the marriage penalty tax. A lot of these other benefits also depended entirely on the state/job/facility and are still required to be granted via other documentation. For example, Florida is a probate state and requires a will regardless of marital status or you have to go through probate (ask me how I know). Medical decisions and adoption seem to be important benefits, but these can also happen without marriage. Survivor's benefits are interesting as well because those don't generally extend past the last owed check, but do still go through probate if you're in a probate state.
- It’s the case unless your household makes over $693,750, in which case you should probably be having an accountant do your taxes, and $6k is a drop in the bucket for you.
  
  For everyone else, the brackets are exactly double the single file, so if you’re making exactly the same amount it’s a wash, and for most couples it means ending in a lower tax bracket.
- I'm curious how that could possibly be the case...
  
  You said in another comment that you were both making $45k. According to this table from the IRS you would each be paying $3,743 as single filers and $7,483 filling jointly. That adds up to a $3 savings when you get married. Not too mention a $6k difference is 80% of the total taxes owed so it would be wild to see that big of a variation caused by filling jointly.
  
  Perhaps there was some tax break or credit you got when you were single that you lost when you got married? In that case, couldn't you just file your taxes separately if there really was a significant difference from filing jointly?
Just to be clear on the first two points, you can designate anyone as your health care proxy. Check your state for specific applicable forms. In general if no one is designated though it will default to a spouse.

Health care proxy is also different than power of attorney, which sometimes people get confused on.
- I’m not sure if America works in the same way but in my country parents can take those. Like you can ask for your unmarried partner to be your NOK but if they fall into a coma or similar then the parents can take that status and block the partner from the hospital.
  
  It doesn’t always happen obviously but it’s happened enough to make the news.

Not an expert, but there is a lot less ground to legally challenge the partnership if a couple is married. If a couple isn't married and one of them suddenly dies without a will or some sort of similar situation, the deceased's family can fight for everything from how their funeral is executed, custody of children, or even control of finances. Obviously it would involve some sort of legal intervention, but marriage would supercede any familial relationships in most cases. I'm not sure how common it is, but I've definitely heard stories about this in gay relationships where estranged family ends up getting legal guardianship of children because the parents weren't officially married and one died.

I'm married and, though I don't think I can numerate all the ways, we are definitely more financially stable than we were when we were just dating. Even just the convenience of being able to do certain legal and medical stuff on my partner's behalf makes a big difference. And that's isn't to say it's for everyone, but it is worth investigating if you have specific concerns.

Are they gay relationships with kids from a previous relationship? Because surely if they adopted them then the living person is still a parent and someone's family can't just take them
- From what I've read, biological parentage can get preference even if the child had been living in another household. You'd think what you said would be the case, but state legal opinions are not a thing I would be super trusting of if I were in that position given how our rights are in a precarious position, to say the least. IANAL, but everyone should write a will. That's better than letting lawyers and judges guess at what you want.

In the US, one big advantage is that you can add your spouse as a dependent under your medical insurance plan you get through your employer. Since we don’t have a public option, this is significantly cheaper compared to them getting their own plan via Obamacare.

It’s possible to add a domestic partner in some states, but they really make it a pain in the ass to do so, if you’re eligible by living in one of those states to begin with.

Can't be compelled to testify against a spouse

Isn't that "you have the right to remain silent etc" stuff on TV shows meant to protect yanks from testifying against anyone they don't want to testify against?
- It's generally intended only as a protection against self incrimination. If prosecutors want to force your testimony, they can grant you limited immunity from prosecution and you can be compelled to testify. Of course it gets way more complicated than that, and I'm not a lawyer.

One benefit is that if you are a US citizen, and your spouse is not, you don't have to wait as many years to apply for citizenship. It can also help your application, as long as you are able to provide sufficient evidence that the relationship is genuine and not just to hasten the application. (Still ungodly expensive and difficult to get citizenship though -- the idea that you "just marry" someone is an utter myth.)

Navigating credit and finances is easier.

It can be easier to get access to be there for each other during a health crisis, and I know my parents have appreciated how much help their son-in-law has given them when they need moral support during medical appointments, etc. Sometimes he's been there for them when I am unable.

My husband and I didn't marry for these reasons, though. We had been in a monogamous co-habitative relationship for five years leading up to the proposal, and it was more for customary/cultural reasons. You're right that the practical benefits are pretty marginal.

It kinda seems like it’s just a way to trap people in relationships, probably traditionally, the woman

It can be, yeah. I think this is an ongoing problem that was even worse in the past. I disagree that it is "just" why people marry, though. I was fortunate to have parents who are married, but it was always clear they were both equal partners in the relationship. Didn't really subscribe to traditional gender roles and such. I'm really lucky.

It's always awkward when someone will ask me, "Who's the woman in the relationship?" Like, what kind of question is that? But yeah, there are a lot of people who unfortunately do have this idea of ownership. Terms like "trophy wife" seem so problematic to me, like wtf bro.

Gonna finish by adding there are many different ways a relationship can look, and marriage is just one of them. I think if it's consensual and equitable, it can be a beautiful thing, but so can many other types of relationships. It made sense for my husband and I, but it probably doesn't make sense to a whole lot of people, and that's fine. I think no one should ever feel pressured to get married or not to get married.

Marriage tax penalties usually affect high income, especially where both spouses make a lot. In many cases taxes go down.

I was married (now divorced) and ours definitely went up. We both made about 45k at the time.
- American marriage tax codes are written to benefit the sort of couple with a stay-at-home-spouse. Having one person without an income (or with a significantly lower income) in the marriage effectively pulls you down in the tax brackets as a whole.
- If you both make around the same, you'll see it go up. If you make vastly different amounts (such as by having only one income), it'll go down.
Mine went down, for sure, but I'm a sole earner.

Mellvar posted a bunch, but ones like immigration and naturalization are huge, as the process for becoming a citizen- especially while of working age- is huge. Having your husband or wife by your side and not limited to trips across the border once a month is really nice.