Should I think with my heart, or below my waist? Even if I hurt someone else...?

This situation I am in is very tricky, and I personally have never been in one like this before. I recently moved to the a new town,which is amazing. My roommate and I have been friends for 14+ years. She has a best friend who is a guy,and we have all hung out fairly regularly for a while now. I never had any kind of attraction to him, but knew that he had some feelings for me. I kept it casual and platonic, because I didn't want to ruin anything with my roomie and for his/her friendship. Until one night... we were at a party and having a great time, we ended up kissing. It felt so great,and I was shocked at how much I enjoyed dancing and smooching on him. Nothing went farther than that. Until fast forward a few months later, we all are drinking and having a fun time, and we end up getting physical. It was the most amazing I have had... It felt pionate,and sensual,and like we knew each others bodies. I ended up developing feelings for him, thinking it was semi-mutual. Our physical relationship continued for a few weeks...Until one evening, I am informed he has a girlfriend...who is from my home town. To top it off, she was coming over to our house in 20 minutes. I felt blindsided,I was very upset, and I couldn't look her in the face,or him. Especially because we had just been together the night before. We didn't talk for a week, and then he came over and we sat down and said our sorries blah blah blah... I told him how I felt about him and the situation. We agreed we could never be together again...5 minutes later we are making out and he tells me how he cant control himself when he is around me. That weekend we spent 2 days together, having the most pionate, crazy,intense ever. We briefly discussed how we shouldn't be doing this, especially on my behalf since I am AWARE of his lady. I just don't know what to do, I cant get him off my mind. He is always in my head, and the thought of us sends chills down my body. I don't want to be "the other woman", that is disrespectful on my part. At the same time, he acts like he cares for me, and tells me all these things about how amazing I am and Im the best he has ever had... Any suggestions? I want him to leave his woman, but would never suggest it. But I want to continue to be physical with him, but am afraid I will only hurt my emotional self. Please help!