Cosmamania
Shuya Yamazaki, the protagonist, is a highschool student. His mother died when he was a kid so his father, a famous archeologist and adventurer, remarried the beautiful maid of the family, Midori.
Shuya has a girlfriend, Wakaba, but they never had sex. One day, he is coming back to home after school when he sees his father and his mother-in-law having sex in the living room! Shocked, he runs to his bedroom.