... decides she wants to have sex and feels she is ready to make that decision, she's going to do it, no matter what you do to try and stop her. You cannot lock her away or send her to a convent.

Well, I guess you could send her to a convent, but...

What you can do is to make sure your children are prepared for it when the time to make the decision comes. Make sure they know the dangers and the risks and make sure they know how to protect themselves and be responsible. Because, if you don't do it, who is?