What sort of rules do you establish with roommates having their girlfriends and boyfriends over?

My roommate just got a new boyfriend and it's getting to a point where she has him over at least three days a week consecutively. I planned on making a rule of if they're over more than three nights a week then they'll have to contribute to rent. I've never met the guy, I'm sure he's nice but I don't want to share a bathroom with someone I didn't sign the lease with.

What sort of rules do you establish with roommates having their girlfriends and boyfriends over?
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