Musuko no Tomodachi ni Okasarete

Based on the erotic game by Anim.

Yoko is happy. She lives a peaceful life with her husband, Masahiko, and their son, Kazushi. But there is one thing she complains... They haven't had sex for a long time. One day, a friend of Kazushi comes to her house. His name is Ryouto. He is handsome and polite. But she doesn't know yet that Ryouto has a secret personality...

(Source: AniDB)