My DD16, has been with her BF since she was 14. She went on the pill at 15 to help with painful periods anyway but I was also conscious that sooner or later things would be happening. I made sure I spoke to her about consent, and not being pressured, and I told her if though she is on the pill they should still use condoms as the pill doesn't protect against STI's etc.
My ex husband went mental at me. Said I was encouraging them to have sex. But the way I see it, if they are going to have sex they are going to have sex, nothing I can do will stop them, so I'd rather know they had a safe space to do it, then somewhere risky like outside where anyone could stumble upon them and anything could happen (I used to work in a sexual assault referral centre so I know exactly what cna happen to young girls in unsafe spaces!)
I made sure that my DD knew, no matter what she could always talk to me about anything without fear of how I would react or judgement etc. I wanted her to know I would always have her back. Because of this she did tell me when they did have sex when she was 15 1/2. And most recently she came and told me, pretty much immediately when they had, had an accident with a condom (they are both almost 17 now so perfectly legal) so we could discuss the need for emergency contraception as she'd missed a few days of her pill a couple of weeks before.
People may disagree with how I've gone about it but I have fostered a relationship of trust and openess with my Daughter, given her a safe space and I know that she will always come to me if she's concerned. If you come down hard on them, or make them feel shame, embarrassment, they are still going to do it but they won't come to you when they need help, won't talk to you about anything and may find themselves in very unsafe situations. So I stand by how I've done things with my DD.