I’d say this really depends on three factors: 1) How comfortable she is with it; 2) How comfortable her folks are with the idea of you two having sex in general; and 3) How long you’re staying.
If your girlfriend isn’t comfortable having sex in her parents’ house, then it’s obviously a no-go. If she is, but her parents are born-again types and/or making you stay in different rooms, then keep your snake in its cage till you leave. Of course, if you’re staying with the ’rents for a long time, that’s sort of the trump card regarding factor No. 2. (Note: Nothing trumps factor No. 1.)
Assuming she’s cool with it and her parents won’t castrate you, go for it—as long as you do it quietly. Choose times of day—before you go to bed, before you’re out of bed in the morning—to make the act less conspicuous. Disappearing after lunch for a “nap” that turns into a liaison is a tacky move around her family. So: If the fam won’t freak and you can be discreet, then yes, you can have sex with your girlfriend at her parents’ house.
Amber Madison is a New York–based therapist, author, and sex columnist. Send your questions to email@example.com