Why not and especially for my daughter or a son because he will be with someone elses daughter.
They are going to do it any way so why not allow them to do it in their own room and safe? The only reason is your own embarrassment and inability to accept your children are growing up.
Or do you think they will not do it if you say no?
They will do it in the back of a car, up against a wall, in a park, woods or where ever they can find.
Few ground rules though.
You can be in your romm but no screwing while we are watching TV.
You come and sit with us for a while to be polite.
Use condoms? Then clean up afterwards.
No orgies.
No animals
AND NO BANGING THE BED HEAD AGAINST THE WALL FOR TWO HOURS OR SCREAMING
BANG ME! BANG ME! BANG ME! BANG ME! 
Leave that to your mother and me.
