Girls Saurus

The story revolves around 16-year-old Shingo Chiryu, who had been beaten and hospitalized for a month by a morbidly obese girl whose naked confession he had rejected. As a result of the encounter, Shingo has developed a fear of all women, called gynophobia. While Shingo tries to hide his fear, the school nurse notices it and advises him to join the boxing club. However, Shingo soon finds out that all of its members are girls, the club's adviser is the school nurse, and one of the club's members, the (now slim) Haruka Nishiharu, is the very girl who had put him in the hospital.
(Source: Wikipedia)