Sex is the result of a God given desire. I have not found anything yet, in the Gospels, about it being a sin. If I understand correctly, the teachings are more about commitment to one person when sex is involved, using marriage as the term to describe this commitment. In my opinion, Jesus is more concerned about the harm caused when folks move from lover to lover, not necessarily the act of sex.
I think I just invited folks to criticize me. It wasn't my intention, though. Any thoughts?