This was not just so that we would keep the human race going, but also because God wanted us to enjoy oneness with a life partner. This unity is actually a very powerful symbol of the way God wants it to be between us and Jesus Christ. Sex is a gift from God. It is not evil. But it is a powerful gift, and it can be abused or perverted.
Sex is like fire.
In your fireplace it can keep your house warm. But if fire gets out of control it can burn your house down. It can even kill you. Sex is like that. Lots of people have now died of sexually transmitted diseases.