I've been meaning to suggest this, myself. The tutorial implies that slaves who have high lust could potentially get intimate with other slaves, however that doesn't seem to have been implemented yet. A "breeding room" or "couples room" could allow two slaves the chance of getting intimate, depending on their lust stats. Perhaps it's even possible for one slave to force another, keeping things more realistic.
I think only one of these rooms should be added to the game, and after being bought, it should be treated with just a bit more luxury than communal. Then maybe an upgrade to the room allows it be considered as good as a private room.