You control one of the most basic instincts of the human race, and you control people. You limit when, how and what type of sex they can have and you control them. Put the fear of god into them by telling them they are going to hell if they disobey. Actually Robert Heinlein said it better than I could:
"Take sex away from people. Make it forbidden, evil, limit it to ritualistic breeding. Force it back into suppressed sadism. Then hand the people a scapegoat to hate. Let them kill a scapegoat occasionally for cathartic release. The mechanism is ages old."
Robert A. Heinlein - "If This Goes On---"
Sound familiar?
SOME hardcore "Christians" really hate heterosexual intercourse. I'm firmly convinced that if humans could find a way to reproduce without intercourse, those "christians" would be ecstatic. Don't EVEN get started on homosex. Otherwise they wouldn't be so concerned about who is doing what, etc...
Don't forget about the "Fall of Man"
http://en.wikipedia.org/wiki/Fall_of_ManThe Fall of Man, or simply the Fall, in Christian doctrine refers to the transition of the first humans from a state of innocent obedience to God, to a state of guilty disobedience to God. In the book of Genesis, Adam and Eve live at first with God in a paradise, but are then deceived or tempted by the serpent to eat fruit from the Tree of Knowledge of Good and Evil, which had been forbidden them by God. After doing so they become ashamed of their nakedness, and God consequently expelled them from paradise. The Fall is not mentioned by name in the Bible, but the story of disobedience and expulsion is recounted in both Testaments in different ways. The Fall can refer to the wider theological inferences for all humankind as a consequence of Eve and Adam's original sin. Examples include the teachings of Paul, Romans 5:12-19 and 1 Cor. 15:21-22.
Some Christians believe the Fall corrupted the entire natural world, including human nature, causing people to be born into original sin, a state from which they cannot attain eternal life without the gracious intervention of God. Protestants hold that Jesus' death was a "ransom" by which man was made forever free from the sin acquired at the Fall, and other denominations believe that this act made it possible for man to be free without necessarily ensuring it. In other religions, such as Judaism, Islam, and Gnosticism no term "The Fall" is recognized and varying interpretations of the Eden narrative are presented.
So the human race fell from paradise because Adam and Eve fucked. That's it in a nutshell.