I agree that it is no business of yours to go into your sons sex life . He is 19.
lectures on contraception, consent etc are way past- and you have to hope the groundwork you did in his years between 10-18 (or whatever) instilled the knowledge he needs to make sensible decisions.
you also would be well placed to remember/know that “impulse control “ is one of the last bits of brain neurological development to happen and we now know it takes until around 23-25 to complete. Hence why the young take so many risks🙄🤷🏼♀️
BUT, it is your home, and all the time he is living there full time or part time if at university, you can have boundaries.
You can say to him his sex life , whilst not your business, is a matter he needs to keep under wraps (🤣) and that includes wrapping his flipping condoms and disposing of them and not leave them lying around in bedroom as it’s 🤢.
you can say he needs to wash it his sheets and keep the mattress clean. similarly the bathroom., kitchen etc.
you can say he does not ever disturb your peace by sounds of his active sex life or even lovely dovey French kissing slurps or even cutesy sickening pet names in your presence 🤢
amd you can say for fire safety purposes you need to know when he has someone staying in his room ahead of it, and you’re not going to go to lengths of having a clocking in/out system for this, just expect him to have courtesy to check it’s ok and inform you
that ultimately you have rights to who gets to come into your house and who doesn’t. And that includes gf and him😉. If she doesn’t follow your house rules, doesn’t clear up after herself and treats it like hotel then you have right to stop her coming over. You advise him to talk her through this, and can produce a “helpful” checklist for her if she wishes 🤣😱. You should also agree re meals, frequency etc- tell him even though he is joined at hip to said gf, you aren’t and have a right to privacy in your home most of the time.
I think people saying you shouldn’t be in his room, is naive. Any mum of late teen/young adult knows that’s a sure path to the mysterious depletion of crockery, mugs and glasses, and an aroma of stale linen and sweaty feet pervading the landing and bathroom. Whilst he needs to be responsible for cleaning his own room etc now, all e while he is living with you, you have rights to enter his room for emergency retrieval and disposal purposes which will, btw, result in some sort of ”fine”.
You don’t have to make it serious OP, just set out some boundaries. That way you’ll be saying you know what’s happening, you accept it even if not delighted it’s under your roof. And, frankly, that’s no different to knowing your random lodgers bf stays over and they have sex in the bed you’ve provided🫣and he is always hanging around so you don’t feel comfortable and relaxed in your own home at all times. It is an imposition. An occasional stay over isn’t a problem, it can be fun, interesting etc. but not a regular habit.