Crushin on a married man...?

Okay, so we are both 21. He has a lot of problems with his wife. They are always fighting and he really isn't happy with her. Now I have been his best friend for going on 5 years or so now. The problem is that although I hate the wife I don't want to mess up his relationship. (Mainly because of his 9 mo. old daughter. ) I thought it was just a crush on my side. Hell, he is head over heals for this chick, yet he married her just for the baby and because her parents pushed it so hard. Two nights ago he was at my house (not uncommon we hang out all the time) and he told me that if it wasn't for her that he was be with me. That I was always better than her, that he wishes I was the baby's mother and not the wife. I feel so much for this guy and now.... now I really just don't know what to do anymore because to be honest, he asked if I could handle being just friends with benefits with him and keep it from the wife. I told him I didn't think I could and I just wanted to stay friends... but my body is winning out over my head. I really want this guy and I can handle being the "other girl". What do I do? Whats wrong with me?!?!?!?! Please only honest advice nothing mean.. I'm trying to stop this from happening.