Darn your good! As a conservative Christian, I couldn't have said it better.
Regarding "because God says so". Hey - that's a pretty good reason. Put yourself in a Christian's shoes for a moment. The creator and sustainer of the universe, the all knowing, all powerful, pure and good God says "don't have sex outside of marriage". You gonna argue with that? I certainly am not going to.
On a more serious note, it does go a little deeper than "God said so". As faster_JR sarcastically notes, we do look to see what God's plan is. KA stated we view sex as a gift not to be opened until the wedding night. The key is the gift part. We believe God created sex for a specific purpose: to foster the "oneness" that marriage demands. Marriage is hard enough as it is. We think God made sex as a uniquely marital bonding activity. But Satan has coopted the benefits that sex brings (physical, emotional, relational). The result is rampant sexual activity outside of marriage. The benefits still exist, but they are not being used as God planned. In a sense, and I think you will see that in the many posts out there, it dilutes the effectiveness of sex within marriage. It is no longer a uniquely marital thing and therefore brings nothing "special" to the marriage.