Am I lesser of a man if I have been with one woman for my entire life?

Horse shit is what you've been told by the people you mentioned.

Your manhood, or anyone's manhood is not based in sex. Period.

Now, there are differences in how you'd go about navigating a long term relationship that involves sex, depending on how well you know yourself sexually (which goes for any other aspect of the self, but that's tangential). But there's nothing wrong, or lesser, with discovery in a partnership rather than spread out over multiple encounters prior to the partnership.

It is true that it takes trying things, repeating them, and experimentation to develop self knowledge. This is true of sex as much as it is with food, music, art, whatever. So there is the risk that as the partnership progresses, the people involved may discover that their sexual needs differ enough to be a problem.

But the truth is that the best sex comes when partners are paying attention to each other, communicating both during sex and in between times. Short term partners, you never have time to develop the kind of in depth knowledge of each other for the sex to be truly stellar the way it can be with someone you spend the time and effort with.

So, you won't even be missing anything that spectacular by having only one partner as long as both of you are willing to work to make sex good. I promise you, the best sex I've ever had was with long term partners.

The only benefit to "playing the field" is giving yourself time to get to know yourself better outside the bedroom. Knowing what kind of deal breakers you have, knowing how you navigate disagreements, learning from mistakes in particular, that is something you can pick up piecemeal just as well as with one partner.

Sex? Not so much. You will not be a better partner in bed just because you've had experience. Know why? Because no two people are exactly alike with sex. Yeah, you have to figure out the basics of where to put things, and that's pretty universal. But what works to satisfy a partner can vary immensely. The kind of stuff that's mostly universal is in books if you don't want to just try things with a partner. Erogenous zones tend to be fairly universal as an example, in that most people with the same type of genitals will mostly respond to touches in some places more than others.

You still get variances even in the mostly universal things though, so you still have to communicate along the sexual journey with a partner.

And, no matter what any idiot or asshole says, neither vaginas nor penises get worn out or otherwise made lesser by use. Yeah, stds exist, but that's a separate issue easily addressed by medical tests. Your GF's body is just as fresh and lovely as it was with her previous partner. She could have been doing gang bangs and you would have no need for jealousy, though that testing would be a very good idea to do before getting started sexually.

No bullshit, y'all will be fine as far as any previous experience goes. Hell, it isn't like a single partner does anything except maybe let a person know how to tab A into slot B so things can get started. That's the only difference between the two of you, so don't let other people's stupidity screw with your head about it.