School Mate 2: Story
One of the 7 mysteries in the school have been the curse by the 3 Jizou sisters. The heads of the 3 Jizou sisters came down by Yukariko and the Hero.
That is the reason that 3 Jizo sisters have become obsessed with Yukariko and the answer to the curse that can make the three sisters wishes come true. For that wish the protagonist did things like this and that.
While cleaning, Yukariko disturbed the three sister's shrine and they are taking revenge on her by taking control of her body and having sex with the protagonist.