Should I be concerned

I've been seeing a guy for a year and a half now. He seems to have very little interest in sex...probably once a week or less. I became very suspicious and it just didn't feel right. It has been this way since we started dating. Yesterday I did something really bad....I went through his emails. I found several emails where he he started out by wishing them a Merry Chritmas and Happy New Year. The emails lead into him asking these girls when they could get together. I asked one girl to his house for drinks and she told him she was in a relationship now and thought it best not to see him. He proceeded to tell her that they should both have enough will power to keep it drinks.

I confronted him about it, and he says he was only joking with her cause he knew she would come. It didn't sound much like joking to me.

Any thoughts?