My children are 19, 17 and 10. Think COVID delayed me having to think about this when they were much younger, thank goodness!
My parents were very strict. When I was 23 and living with my boyfriend/ now DH, we still had to be in separate rooms when I went home. DH’s family had his 13 year old sister sharing hotel room with 13 year old boyfriend - then DH, now EXH.
So I was going for a middle ground, I think. When DS was in y13 he had serious girlfriend (also 17) . We let her stay over in same room - but made sure her mum knew. Not sure I would have been as keen in y12. And def not under 16 and age of consent.
Yes, they will probably have sex anyway, but I don’t have to make it easier. And 13 is still very much a child, whether they feel like that or not.
When my kids try the ‘everybody else’s mum’ line, I channel my mum and use the ‘well, I am not everyone else’s mum’ and ‘if everyone else jumped off a cliff, would you?’. Ah, golden oldies…
You need to be the parent here, and help your daughter establish boundaries.