Forgive me for Sex at Home, Father! (1998)

Hidekazu is invited to the house of his young lover Ami. Sex immediately begins in her room, but Ami's father-in-law watches the whole thing. Later, Hidekazu is called by his father-in-law and visits Ami's house again, and is captivated by Ami's bewitching mother, Chifuyu. It's all a trick of Ami's doting father-in-law.