My best friend is a married man. He just told me that he had sex with another married man. This has only happened once, he said. Does that make him gay or just curious and open in his sexuality. He's been married for over 10 years.

Well, there are lots of men that experiment. But, what it makes him is a cheater. He isn't being fair to himself or his wife. I feel bad for her.

This makes him bi and is something he should tell him wife. Cheating is cheating and no matter who he has sex with him and the other guy are wrong and most likely on the down low.

What is his purpose for getting you involve in this? He is the only one knows the answers to his behavior, Now this secret would cause marital discord plus it will become a bench mark in your relationship with him too.

Well, I think he was curious and his nevres/curiosity got the best of him, and he couldn't think rationally. I think all men have these curious thoughts at one time or another. And I think that he confided in you because he is scared and needs somebody to talk to... I think you should tell him that he should seriously think about tell his wife because secret grow and coming from him would be much much better than coming from someone else. And an understanding wife will forgive him and if he truly wants to stay with her and she loves him, they will stay together, even though there will be extreme awkwardness at the beginning.

The act itself doesn't necessarily make him gay or bi. Our sexual orientation is based on how we define ourselves, and not necessarily on our behaviors.

What he is is someone who cheated on his wife. He is clearly feeling guilty about it if he decided to tlak with you. I recommend couples counseling, because there is more going on here than the sexual liaison.

It makes him unfaithful.

No not the only thing that it makes him, it could also make him HIV positive, so he needs to get himself checked. Also I myself think he needs to tell his wife so that she can protect herself if she decides to stay with him.

Being HIV is thrown around a lot in the gay community, I really don't like the stereotype.
It's not just this that he needs to be concerned about it is a lot of other STD's. Who's to say that he has not cheated on his wife more than once and only told you about the one occasion?

I would be bringing it up if he mentions anything to you again.
I feel so sorry for his wife.

As others have said, who he cheated with is not as important as the fact that he cheated in the first place.
His wife should know, although try to get him to confess to her himself.

It sounds like he has been unfaithful but when did this happen? Before or during the marriage? If it's during then he may well have cheated on his wife on more than one occasion. If it's before then he was either just curious or (more likely if he is asking himself if he's gay) that he is bisexual. If you are close friends with his wife then it's unfair for him to confide unfaithfulness in you as your loyalties are divided and you may wish to ask him to stop.