Cosmamania

Shuya Yamazaki, the protagonist, is a high school 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've never had sex. One day, he is coming back to home after school when he sees his father and his stepmother having sex in the living room! Shocked, he runs to his bedroom.

The next day is Wakaba's birthday. As a gift she wants to finally have sex with Shuya. But because he was still shocked for what he saw the day before, he couldn't do it, making her mad.

Depressed, Shuya tries to talk with his father, but he's off on another of his adventures. Without choice, he asks Midori advice.

"Maybe you don't have confidence and experience enough with women..." - she says.

And so, Midori decides to train her stepson in the arts of sex an teach him how to satisfy a woman...

[Translated from Getchu.com]