As the story goes, there was once an
MIT biology major whose
UROP or lab class required her to spend many hours a day sexing toads.
Toad sexing is an extremely smelly and unpleasant process, since determining the gender of your average toad involves examining the inside, not the outside. Since the job was unpleasant enough as it was, she tried to take her work home with her, much to the dismay of her roommate.
Now, her roommate was a hacker, who spent much of her time exploring the underside of the MIT campus. After experiencing firsthand the joys of toad sexing, she changed her sign-in to "No Toad Sexing", so that no future biology majors could use the tombs and tunnels she explored for this particularly unpleasant practice.
Today, "No Toad Sexing" has been picked up by several other unknown hackers, and has been spotted not only all around MIT but even as far away as university steam tunnels in Israel. There has also, inevitably, been a sort of quiet pro-bio protest movement; someone has taken it upon themself to ensure that if a new location is found, its walls read "Toad Sexing Allowed".