Can you point me to the verse(s) that say sex outside of marriage is a sin ?

There aren't any, near as I can tell. I was referring to adultery in my post.

As for sex before marriage, there wouldn't be reference to it in the bible because in Jewish culture of the time there wasn't a question that sex before marriage was a bad thing. So I think the only way you'd see reference to it would be if the norm needed to be challenged. However, arguments from silence are bad and so I won't try to make one. I will say that based on what I perceive the marriage covenant to be (from scripture), I think extreme emotional and physical intimacy should be reserved for that relationship alone. However, I must admit that there is a fair degree of my interpretation going into this, and I can understand how someone might come to a different conclusion.