This sounds like a really horrible experience. You kept your calm a little better than I would have. I am guessing it wasn't a hedonist resort. Can't tell by your slight description of the place whether it was an adult only or family friendly. Either way, if it is not hedonist, then there should be no sexual displays in public. Maybe the other guy was used to getting away with it in public because the managers were lax on the rules. Or other people were scared of reporting him.

I suggest that if this ever happens again that you let first let him know that you are not interested. If he kept saying that he is gay then just let him know that you are straight and you are not interested. If he keeps persisting then tell his partner that he is talking with other guys and making sexual advances. By not saying anything to him then in his mind it was ok to keep trying. I am not blaming you for his actions, but this made it not seem bad in his mind. And the drunker he got, the more he wanted to press it. If anything, there should be a way to get a hold of the manager and report the incident. Especially if 2 people are sexual in public like that.

I usually go to a family friendly resort so that there is less chance of sexual stuff in public. Make sure to read the rules of the place before you go and make sure they aren't too strict though. The closest nudist spot to me is family friendly, but their rules are a bit too strict for me. And it really isn't that big. Married people can't even hold hands. Although I usually go alone, I believe that is a bit much. You shouldn't let this interaction change your mind about being a nudist. Just try to be a little more assertive earlier if something like this were to ever happen again.