After you complete the game and get the foursome scene, it would make sense if the characters are scattered around the house randomly and may appear in the same room while you are having sex with a different character, as they are all comfortable with sharing at this point.

I think this would be relatively simple change to make and it would add a lot to the game. It would also be nice if some of the dialogue was changed at this point to reflect that they no longer care about getting caught.