Speaking as a mother of 3 ( 2 daughters) and having been a teen myself. I would have to say if she is asking, telling her no will not stop her from having sex. She has already decided to or has done it. I would feel more comfortable saying I'm so proud you came to me, this is a huge desicion, let's talk about why you feel the need. Sex is inevitable. Unfortunately parents can't control every aspect of a teens life. Yes, they will have sex in 30 min right after school if they think they can't get out at night. They will find a way. Telling them no will not stop them. Your best defense is to be open and honest. Make sure they feel like they can come to you. If my daughter came to me I would be so relieved the she felt like she could do that. I would discourage it at all costs but I would not forbid her. That would make her run as fast as she could to someone else. My daughter wears her promise ring. She is only 12 but knows what can happen if she makes the wrong choice. We've talked openly about this. Parent's don't shut the door, you may never get it opened again.