Shocking ~Juunin Jusshoku Zenketsu Ryoujoku~

Tentaku is a monster with a lot of tentacles who has been injured in a recent fight with a shrine maiden. He takes refuge in the crotch of Negame Kouichi, a boy with an ordinary life.
Tentaku takes over Kouichi's mind, and with his loins transformed into a tentacle monster, Kouichi repeatedly assaults various women in order to heal Tentaku's wounds. In the process, he slowly transforms into a monster himself.

[From DLsite English]