My rule for children, male or female, is that they can sleep together in my home if they are living together in their own home. Period, end of discussion. I've told them that, prior to that condition, I consider it rude and uncouth of them even to ask me if they can do it--they should know better (and they do--none have ever had to ask me).
One time, about 5 years ago, I came into the living room in the wee hours and found that my late-teens daughter's boyfriend, also late-teens and a very nice kid (now her twentysomething live-in fiance'), had moved from the sofa to her inflatable queen-size mattress on the floor and was sleeping with her there. I nudged him with my toe, got his attention, and pointed at the sofa and snapped my fingers--no words were necessary. He got up and moved back to the sofa and never tried this again.
I should add that, as a divorced Dad, I live in a 1-bedroom condo (in a highrise in a big American city) that can sleep as many as 6 via sectional sofa, inflatable mattress, king-size bed in my bedroom, summertime hammock on one of the balconies, etc. But the only privacy to be had is in my bedroom, and that's where *I* sleep. So it's not like it's really an issue.