In my opinion, sex before marriage causes great harm because the relationship doesn't yet have the proper foundation; it reduces a beautiful gift to a mere lustful physical act. Before marriage, there is no commitment involved. There must be commitment, (marriage), if the sex is to fulfill the purpose for which God gave it to us. Sex, when exercised within the confines that the Lord has specified, is amazing, beautiful, wonderful! It allows us to express our love with the one to whom we have committed the rest of our lives. It is also a beautiful picture of the relationship between God and His people. 

Thanks for everything you do, Dr. Laura! I was one of your students when you were a professor at USC. Boy, have you come a long way and that's a compliment! 

Sean