Cheating is cheating, no matter who it's with. If he had curiosities about his sexuality, he should have explored that when he wasn't in a relationship. Or it could be that he's been forcing himself to play a part very different from who he really is, if you get what I'm saying. I'd probably break up with him, but would first ask why he did that with a guy. Either way, it isn't fair to you.
Hmm its weird when girls cheat with girls that they barely get reprimanded yet if they cheat with a dude its often terrible outcomes. I guess that depending on your stance with infidelity its entirely up to you. It wasn't with another girl but it was with a guy.