Old Post: I briefly mentioned the equivalency of marriage and sex in this post.
So you can see that I'm not the only Christian who thinks that the Bible treats sex and marriage as synonymous, I'll point you toward a sermon given by Daniel Harrell, the Associate Pastor of my former church, Park Street in Boston. This was a controversial sermon, and he didn't attempt to back it up Biblically in the sermon, although he does discuss it in more detail here. Personally, I would be much happier if he had given more Biblical explication. I will say, however, that Pastor Harrell is not responsible for my thinking on the matter, which I came to based on my own study of the Old Testament prior to hearing his sermon. I may discuss the Biblical verses more thoroughly at another point.
Update: Park Street Church is located in Boston, Massachusetts (we're not all liberals there). I added that above.