Hitting her is totally unacceptable. Maybe you should try and be her friend in the situation. Get her on birth control. If you "forbid" sex, she is just going to do it behind your back. Instead teach her safe sex.
It's partly your fault that you do not have trust in her. You need to be her friend when it comes to this and not punish her for doing it. She is going to do what she wants to do. I think you owe her an apology. I don't believe in children being hit above the age of 10.. but some parents discipline differently than others.
You absolutely CANNOT punish her for this. It's nature. It's bound to happen. I hope you didn't expect her to come to you first and "ask" you if she was "allowed" to have sex.
If you don't go about this in a friendly manner, she is only going to turn against you.
