The Distance Between Similar People
In the second episode of MM! we’re given a larger picture regarding the connection between Yuuno and Tarou. Yuuno, Tarou’s middle school friend, is the one that caused the sexual masochism fetish that he has right now by slapping him in back in middle school.
Aired 09/10/2010EP 2 - The Distance Between Similar People

MM!
There are twisted tales and twisted tales, but few are as twisted as poor Sado's, who's just realized that he actually likes being made miserable. Of course, knowing that only makes him more miserable, which in turn... well, you get the idea. Desperate to break the circle, Sado volunteers for a special club where he hopes he can work through his issues only to discover that the other members have equally... complex... issues to deal with.
For example, the hyper-aggressive club president Isurugi not only has a violent fear of cats, but also believes herself to be a god! Then there's Yuno, who's terrified of men; the Nurse, who forces other people to perform cosplay; and Hayama, Sado's best friend and a compulsive cross-dresser, who's also the girl that Sado is infatuated with.
(Source: Sentai Filmworks)