Teenagers having sex?

Sh
Piggybacking off of another post, I have a question that truly mystifies me: Why is it better for teenagers to have sex in your home (as their parent) than out in "the streets?" 
This is for the parents who allow teens to have sleepovers with their boyfriend or girlfriend. I truly don't get providing sanctuary for teenagers to have sex in your home. Do you want them to become teen moms and dads?