Let's try to figure out a few. Because sex is the awesome link human beings have to creating other human beings. Sex is awesome, when you think about it. There's always the chance that from a coupling a child will come to be. You know, the miracle of tiny, little grasping fingers and a red, scrunched up face, and all the parts that work.
Babies come from sex.
And since we place such a high value on human life, then we don't degrade or devalue what causes human life. We guard and protect it. We build fences around it for the sake of babies. Cute, vulnerable babies. Oh, yes, I've heard about birth control.