Although in the case of the OP's DS and his GF they're already sexually active, so she wouldn't be preventing them having sex at this tender age.
So was I. It wasn't about her putting the horses back after they had bolted. It was about me having a something to anchor a retreat from sexual activity on, or a way to avoid escaltion at the very least. Or at least to feel like I could limit it to my preferred pace, frequencey and degree by using home as a "veeto zone".
Back then there was an attitude amoung my peers that once you'd done the deed your excuse to avoid it with that person or the next person was gone. With mum seeming to agree that there was no basis for even trying to do something to stop it, I felt like my mother had come down on the side of my peers view and thus confirmed it's validity.
On a very "teenage thinking" level home was my safe zone. I could say no there becuase "mum will go mad", and I could play the "I have to go home, not allowed out today" card to avoid contact extending beyond my confort zone by having contact limited to inside the safe zone. But then home stopped being safe cos the kybosh had been removed.
It is hard to explain it as a grown up. My teenage head when I try to get back into now was all one big conflictual mess...and far too worried about what everybody else thought I should rather than just doing (or not doing) what I wanted.
The best way to describe I suppose is it felt like my one layer of protection was whipped off, by my mum. She didn't mean to, god knows I never tried to explain it and out of daft teenage pride/dignity felt almost obliged to follow through and take advantage of the new leeway so there was nothing in my behavoir that would have suggested to her the "whipping off of protective layer" thing. Not without gaining a sudden ability to read my mind anyway. Which is asking a bit much of a parent. 