Amanee!: Tomodachinchi de Konna Koto ni Naru Nante!

Based on the same name adult manga by Shiomaneki. Ryouichi goes to his friend Yuu's place to play video games, like they do sometimes. It turns out that Yuu's aunt Yui has come to visit, and she joins them. But it soon becomes clear that it's not a video game she wants to play with Ryouichi... (Source: AniDB)