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Leave it. As long as they are being safe, thats all you can do. We were all 17 ones :)
I was doing far worse at 17. If all he's doing is sleepovers, I really wouldn't be worried about him. Let him be.
Did his Dad have the same feelings as you about it?
I would tell the dad to ask him about it
They're 17. Better safely at your house than elsewhere! Also, as you're in the UK they're over the over the age of consent so aren't doing anything illegal. I'd drop it tbh!
Thanks everyone
Thats very disrespectful of him.
Why let him at all if you are uncomfortable with it? Tell him you are not ok with it or set certain boundaries when they are having sleepovers, and explain why. Your house, your rules. I personally wouldn't be ok with it if my son or daughter wanted sleepovers with their bf/gfs and I would explain it's because I don't want them having a sexual relationship until they are married. I appreciate there is every possibility that they are having sex anyway but there is also the chance he or she has listened to my advice. I'm not going to let them think I condone it by allowing sleepovers. Personally I think it's giving them too much free rein and could be problematic in the longterm.
Leave it, it's better to be safe under your roof