Obviously this is something that really bothers him, and he wants someone to still accept him, knowing his deepest, darkest secret and his greatest shame. It's a heavy burden to carry.

On one hand, what he did is reprehensible, regardless of his reasoning. He may now have it programmed into his brain, and he could do something even worse in the future. On the other, people can change. People can overcome their horrible pasts and learn to live with themselves, and be better people because of it. Just because he did something horrible once, doesn't mean he'd even consider doing it again.

I think one-on-one therapy for him, and couples therapy for both of you would be very helpful in this situation. He needs to come to terms with his own past, learn to forgive himself, and move forward. As a couple you need to evaluate things. Maybe it's not something you can look past. Maybe he needs time to work through his guilt and grief and get through it, and a relationship may hinder his progress. Maybe it's something you can work through together and strengthen your bond.

Either way, good luck. It's not an easy situation to be in.