Yabanjin no Nekutai.

Motozo Imamura, a.k.a. Gen, grew up in an upper-class family, but he never went to school and spent his days playing around. One day, he walked into an orgy of male-female sexual misconduct, and when he dropped off Roze, whom he had met there, he saw her sister, Kaoru, forgetting her impurity. One day, he went to an orgy where he met Roze, and he could not forget the purity of her older sister, Kaoru. A few days later, Gen went to the party again and was mistaken for the boss's brother.