Cmon, have a bit of class: do it in the back seat of your car.
This guy is probably still at school (well, I'm guessing so as he still lives with his parents), so perhaps he can't drive yet, so perhaps he doesn't have a car.
Just make sure your parents have the courtesy to knock and not enter when you're in your room with her (which they should anyway!) If you have wandering siblings, barricade the door some way! It shouldn't be too much of a problem really.
Buy a lock, fix it to your door on your bedroom, use it. Or give your parents a tenner to go to the cinema for a couple of hours or something :P
I think its the fact them knowing they're going to have sex that is the problem, so buying a lock is going to turn their attention just because they cant see them doing it,