Confused And Frustrated

I have been in a relationship with a guy for almost two years now and I am now FURIOUS after discovering that my partner, in his previous relationship, had an affair with his girlfriends daughter! His own stepdaughter, with whom he was living. Apparently, it was consensual and not forced, and she was of legal age, but it stills seems to me to be such an abuse of his position and his trust to be positively revolting. I have confronted him with it, he has admitted the fact and said that it was a mistake, and that he would never do something like that again. But now I can't look at him without thinking about what he might do to my own daughter, who is only ten years old. Yet the worst part is that I still love him. I don't know what to do, but I think I will just have to throw him out and cut him out of my life. I feel so frustrated, like the last two years of my life have been wasted and based on a lie.