this has nothing at all to do with you making a judgment about his sexuality, it doesn't even need to be the focus of the dialog. The fact that your partner is having casual sex secretly with other people is the problem regardless of the orientation of said encounters.

I would not keep this to yourself for one second. In addition to the idea he's stupid for cheating in the first place it's his own darned fault for leaving the email logged in on your computer so you didn't even have to snoop to find out. You're free to mention what you saw, I think. But you dont' even need to justify yourself. walk away. Just tell him matter-of-fact that he's a cheater and you want someone that's faithful and leave it at that.

I said walk. I meant RUN.