@continentallentil
Sorry I might not have been fully clear.
I appreciate you cannot physically stop the act of sex!
But I would argue, broadly, you can work very hard at creating the situations where it can be prevented.
Largely with a huge amount of dialogue, agreeing boundaries, shared understanding of why sex at a certain age is not appropriate.
I know that situation has moved on a bit for OP, as her DS is already discussing it. I still think really clear, direct conversations about why it should not happen, coupled with proactive involvement in his day to day life, can be effective.
The idea of holding up my hands & going - I wish he wasn't but .., to a child having sex is anathema to me
(I should also say I know that other parents will view having sex (at a slightly older age than 14) as something they're ok with. I am not in that camp so clearly that influences my views! I accept others differ & that's ok too)