Even though it could be hard, I think you may want to try understanding your friend's situation before jumping to any conclusions about what precisely his behavior means. Coming out of the closet may have been very difficult for him.

It is reasonable to feel betrayed when you are the last person to know about something. It is also natural to feel that a close friend should have entrusted you with a "secret" before just any old stranger. However, it is probably a good idea to hold off on getting really angry with your friend for the time being. Not only must he deal with some larger social and cultural hostility about his sexual preference, but there is also the possibility that his family and closest friends could reject him as well. His worries about how you, an obviously important person in his life, might react may have kept your friend from talking to you first. Maybe he even thought you knew, for some reason.

More than likely, your friend was probably having a hard time dealing with this himself, and my advice is to cut him some slack. He didn't tell you first, but he did tell you, and he probably needs the help, support and understanding of a good friend right now. In the near future, you might want to ask him why he never told you this earlier. But I think you might want to give him a bit of time.