This is very true.
But from OP's posts it sounds like she and her son are from a culture where sex before marriage is commonplace and expected, so it probably makes most sense to respond with these cultural expectations in mind.
My priority would be to make my child as safe as possible, and I personally believe this is best furthered by promoting honest communication.
If you show that you disapprove of your son having sex, and make active attempts to limit his opportunities to do so, won't he just very likely stop talking to you about it? And potentially seek less safe opportunities for sex (in the back of a car, etc).
Therefore I would speak to him honestly, share my concerns, attempt to persuade him to wait (if you think this is preferable, knowing him as an individual, his relationship etc). But ultimately I would allow the sleepover as the least worst option, if these young people are determined to have sex.
(I also wouldn't break my heart over it. While 16 is on the young side to become sexually active, at least it's in the context of a relationship with someone of a similar age, so less likely to involve coercive behaviour on either side. Plenty of us became sexually active at around this age and have grown into comparatively stable, well adjusted adults! Pregnancy is a lot less likely if you keep the conversation about contraceptives open and encourage him to tell you straight away if she DOES have a scare, so that you can talk to them both about options.)
This may appall a lot of you on here, but I keep a jar of condoms in plain view in my bathroom, and my daughter knows she is welcome to help herself for her or for friends, even though she is not yet 16 (and is currently in a relationship with another girl, and not sexually active at all as far as I am aware bar "making out") . I speak to my daughter openly about sex and she knows full well I would infinitely rather she wait until at the very least 16, and preferably longer. BUT if she, or one of her friends, chooses to have sex, in my experience they won't wait for lack of a condom. So I'd far rather they had it with one, than without!.