I have this situation with DS1. He is about to be 16 and his girlfriend already is. They have been together for about 3 months.
I don't like driving at night, especially in the winter, so I said I was happy for her to stay over on the sofa in the spare room (we don't have a spare bed).
She did creep across the landing in the middle of the night and I had to have a chat with DS about how I did not feel that was appropriate.
But they do spend time in his room during the day, and he has a double bed so it's a very grey area.
However, I do think 'sleeping together' is a more intimate act than sex in some respects and don't feel comfortable saying it's fine.
But if they're going to have sex, they're going to have sex. It would be more useful to never leave them in the house alone together if that is what you're worried about.
Also, having strict parents as a teenager didn't really prevent much, it just meant that there was a lot more activity in cars, parks, random places and I think I might have allowed myself much higher standards if I was allowed to be more comfortable at home.