Your sexual relationship is healthy, and your overall relationship is healthy. Your sexual and overall relationships are so healthy, in fact, that he was comfortable telling you something very personal that he probably hasn’t told many people.
Men who identify themselves as heterosexual don’t, generally, have sex with other men. But it’s certainly not unheard of, and sexual orientation is a bit more complex than “Straight or Gay.” There’s nothing wrong with your boyfriend, and there’s nothing wrong with your relationship. His past is his past and now you’re the only person he’s sleeping with… hopefully.
Don’t be any more concerned about him sneaking off to have sex with a man than you would be about him sneaking off to have sex with a woman. If he does either of those things, he’d be a cheater, and that’s bad, but he’s not (nor was he ever) some horrible sexual deviant.
Enjoy your relationship, enjoy the fact that he loves you, and plan for a happy future.