Some years back, over winter break, in the middle of the morning the door to my younger son's room opened and out popped son and his girlfriend. We (my husband and I) were quite taken aback but simply decided to offer coffee to them.
A couple of years later I happened to be speaking to the girlfriend, and she told me that my son had promised her he'd checked in with us and we were fine with her spending the night there, and when she came out of the room she could tell we had no idea.
Here's what I want to convey. Your kids will have their own life, which may well include sex. If you don't happen to agree, well too bad. Get over it. And I'm going to guess that most of those who claim to be opposed to premarital sex were themselves sexually active before marriage. What's the word I'm looking for? Oh, yes. Hypocrisy.
Sex before marriage is hardly new. Get over it, and worry a bit more about unplanned pregnancy, especially if you (and I'll invoke hypocrisy here) oppose abortion. Adopt otherwise unwanted children, otherwise STFU.