1 Corinthians 6:17

But he that is joined unto the Lord is one spirit (KJV)

Then: Paul works with the principle of "Don't just give the guy a fish." It's a whole lot better to "Give the guy a fish net." The fish is a behavioral rule, but this verse is the net, the principle explaining an action (Wright, p. 73-4). When partners in sexual experiences unite, they completely use the body and mind. Union is not incidental or unintentional. Joining with the Lord is the same. Paul wants the Christians to understand that when they are joined in spirit with the Lord, they have full access to his mind. The two become one in body, mind, and spirit.

Now: The popular axiom is that teens will explore sex, no matter what. The opposing axiom is "Abstinence works every time it's tried." We're stuck in a generation that has lost out on the sacredness of the human spirit, so the rule doesn't make sense. Everyone needs the principle. God keeps calling; very few hear him.