The Bible on pre-marital sex

Where does the Bible actually say that pre-marital sex is wrong? My parents have always taught me that the Bible commands us not to have sex before marriage. Now that I'm old enough to think for myself, I'm having serious doubts about this. I haven't run across anything in Scripture to support their views. In fact, I can't even find any biblical basis for "traditional" marriage! Isaac and Rebekah didn't have a church wedding. Apparently they just slept together and that was it!