Another game idea:
You visit a couple and the girl is quite playful. From time to time the boy is busy and she secretly plays with you e.g. by flashing.
You have to react enough to keep her in the playful mood but don't exaggerate or she will stop or you'll be detected. If done right she will go further with each situation and the final target is to fuck her. If you are too fast and you're detected the friend will request you both to stop and even throw you out.
That would even allow a branching story line and could be very dynamic.