In university halls of residence. That thing when you share a room...
...and one of you has to not go in because the other is having 'Sexy Time' and has left some sort of sign on the door. This used to be a regular occurance of a mate of mine when I was in London. He studied computer sciences (or some such, I spent most of my time in London drunk) and I shall call him Hugh, for 'twas his name (Ahh the laughter we had at his expense, many, many jokes he had heard since he was about three). Anyway, his room-mate was a stoner called 'Mutch' (He was actually called Jason Starskey but the obvious reference to the seventies cop show meant that he ultimately became known as Mutch through mispronouncing it whilst drunk/high) who was on the same course as he and, by all accounts, was beating off the lady-folk with a poo-covered stick. This annoyed the Heck out of Hugh who would have to stand in the hallway or bugger off to the nearest bar until Mutch and lady-friend had finished. It became well-established fact* that this is why, after a few months, Hugh came out of the closet to us all. Thus it began the legend, 'What leaves Hugh cold, just as Mutch has it. Turns everyone else son.'

*i.e. humerous lie
( Funkenschlag still likes dogs more than cats, Thu 15 Oct 2009, 17:48, 1 reply)

Perhaps
I should add, as a disclaimer, it's been a while since there's been a decently pun-worthy question of the week so I'm sorry that there was the age-old situation of 'Too much setup, not enough joke' in this one. I'll try harder.
( Funkenschlag still likes dogs more than cats, Thu 15 Oct 2009, 17:52, closed)