Futanari Seisaikan

Synopsis

At first glance, the shy and studious Serika Miyagi seems to be a normal high school girl. But in reality, ever since she accidentally wore the underwear of her roommate, Mayu Ootori, she became something much more bizarre. Behind the facade of a young and cheerful girl lurks the persona of a lustful and perverted Serika who possess both male and female genitalia. As a result of this, she has an unquenchable libido, and finds intense pleasure from stealing Mayu's underwear and masturbating to it. However, Mayu has caught on to Serika's indecent deeds, and has gathered the help of two friends—the disdainful Yuuko Natsuki and the mature Yurie Tomizaki—in order to deliver appropriate punishment. But punishment comes in many forms, and they seem to be fixated on one rather "inappropriate" method… (Source: MyAnimeList)