"It wasn't a case of giving her permission to sleep around but you can't lock a young girl in her bedroom 24/7," is the (feeble) explanation.
There was a girl in my jr high school (middle school) whose mom did that. The girl acted on the "protection" and became "everybody's" girl. She thought she was having a good time, but she was simply used and abused, passed around from boy to boy. It was a terrible, tragic thing.
Some girls had sex, some didn't. What they didn't have was babies. My entire high school experience, in a school with over 2000 students, there was one pregnant girl (top ten in her class, got married to the father).
When i see these pregnant teens today, often with a baby already, i think "my god, how can you be too lazy to take birth control?". Sadly, it seems that pregnancy is just viewed as a condition, and a child is something you deal with rather than something you cherish.