Well, he cheated on you. That's your bottom line.
I know what it feels like to find out an ex was gay all along, but it's something quite different, I imagine, to find out your boyfriend has been out and about with someone of the same sex. It was very unsettling for me just to know that I missed all the signs (in my case, there were BIG, neon, light-up signs that I ignored.)
It seems to me that your boyfriend has a lot of growing up to do. He has to find himself before he can be happy with someone else, but is scared to lose you.
Can you trust him not to cheat again? I think that's the question you have to ask yourself, regardless of which gender he's cheating with.