Honestly, I would explain to him that if you choose "damnation" then that is your choice and that if he truly loves you he will cherish the time you have together and accept you for who you are. Bisexual or not you are monogamous to him and that should be enough.

I wouldn't suggest lying to him and saying that you're straight, if I'm right that will only add further anxiety and possibly guilt about not being completely honest. If it were me I'd end up growing continuously frustrated with the knowledge that I cannot truly just be myself with my significant other. (I had a somewhat similar experience with an ex not accepting my religion being different than his own...he wanted me to lie to his family saying I was catholic whilst I'm actually pagan/wiccan and it just ended up making me feel trapped and I'm honestly quite a free spirit.)