You sound like a very intelligent person, so I'm sure you know that honesty is necessary here. THe problem is, in this type of situation there are several ways of being honest. Though I haven't dealt with this kind of thing myself, religion and sexuality issues like this always interest me so I do think about it. You can't talk someone out of aspects of their faith, for example he may always believe bisexuality is wrong. But, you could compromise. Here's my idea.

From how your post sounds I'm going to assume you wouldn't start dating a woman (or anyone else for that matter) while you're with him. So he doesn't have to worry about that. Even though he believes it is wrong, you don't share the same beliefs and maybe there are aspects of his beliefs you don't particularly like either. My point is, he may learn to not necessarily accept it, but deal with it. Maybe someday he'll accept it too. If you truly want to be together I believe you can find a way to compromise. Good luck