If they are anything like I was as a teenager allowing them to share in the parent's house is a way of guaranteeing they won't have sex. The idea of it with my parents just down the landing - no no no.
And when my teenage daughter asked I decided that getting her home safely was more important than any qualms I might have. And once her boyfriend had escorted her home at some ungodly hour it seemed only right that he wasn't then expected to proceed home on his own but was safe at ours.