When my DD had her first BF (she was nearly 16,he was nearly 18 when they met), my starting point was - if they're going to have sex, there's nothing I can do to stop that. I either deal with it or go into denial and make it as difficult as possible, which would more than likely result in them having sex in some random, uncomfortable location, probably in not very safe circumstances.
Once I got my head round that, my outlook completely shifted and I realised the best thing I could do was make sure my DD was informed, confident to make her choices and make sure she was safe, happy and secure. I also made sure she completely understood the potential consequences and the reality that the boy/man can choose to walk away and she cannot. I made sure she had condoms, whether she thought she was going to have sex or not.
And the agreement we reached, after many discussions as to why/please no/ you can't etc, was
1.when he initially came round, they'd have to sit with me downstairs as we only have one reception room
2.Once I got to know him a little, after a few visits, I'd have 'responsibility of sex' talk with him, as well as the 'real sex isn't what you've seen online, and porn is a very different thing to relationship sex' talk (she'd forewarned him), as I had no idea what he had /hadn't discussed with his parents and it wouldn't be him risking pregnancy.
I taught online safety to children aged 6 - 19 as part of my job for 8 yrs and I'd say 80% of boys and a significant %age of girls get their sex ed from full on hard core online porn (most of which is for the male audience so the women are basically sex objects), because school curriculum is severely lacking and parents are too embarrassed. This can result in a really skewed, fucked up view of sex and intimacy.
It was a little uncomfortable but I'd rather that than a teen pregnancy.
Sex is sex. Most of us do it. Some start earlier than they should. Some are completely unprepared. Some are terrified of it - sometimes that's entirely the fault of parents.
As long as it's consensual, they know what they're doing and there's no 'power imbalance' eg big age difference, I wouldn't worry about it.
DD is an only child with no family her age so had little choice but to confide in me, unfortunately for her! She told me when she was thinking she may have sex. She told me when she did have sex. I just made sure she was responsible, and comfortable with her choice etc. I'd rather she was having sex in my house, in a clean environment where she felt safe and in control.
2 short years later, she's at uni living away for home doing what she chooses.