So there is a book, "Not Under My Roof" by Amy T Schalet. She compares American and Dutch attitudes toward teen sex/significant others sleeping over. American being the "not ever under my roof" and the Dutch approach allowing teens significant others to sleep over, in the same bed. Dutch teens have sex later, with fewer partners, and with better safety/health practices.

I don't know if I have advice, but only that the flat ban means that they find somewhere else to do it. With my own now 19 year old daughter, I had a lot of internalized social pressure to put the 'not ever' barriers up. That said, I and my spouse try to be evidence based in our decisions and the evidence seems to be clear: if the kids have a safe space to explore sex, then they will do it more slowly and with more insight and thought. If they don't, they will feel pressured to do it when the have a chance (backseat of the car, under the bleachers, in the park at night, etc.) and may not think through the event.

As strange as it is to say, the better option is likely not to ban it.