Welcome to Psych Central confused kitten. I am sorry you feel confused about the situation. It is surprising to me your boyfriend brought it up and then said how wonderful it was. This sounds like a not so subtle form of mental abuse on you. It is certainly disrespectful and an affront to the sanctity of your relationship.

It apparently is not a totally unique situation as it happens to others.
Psych Central - Search results for Son having sex with mother

I would get a therapist to help you sort out these details and figure out where your relationship is at with your bf.

There are lots of compassionate people here that can make the load lighter by sharing and caring. Feel free to participate actively at Psych Central.
http://forums.psychcentral.com

Please feel free to private message me or any of the Community Liaisons by left clicking on the name in blue to the left of their post) for questions or just to share.