Amaeta na Osananajimi wa Ore de Otona ni Natteiku

Asahi's a high school teacher. He's always treated his childhood friend Hinata like a younger brother. Ever since they met, the angelic Hinata's considered Asahi his personal hero. On Hinata's twentieth birthday, though, he sneaks into Asahi's bedroom hoping to fulfill a dirty promise his older brother made. Hinata's become one hell of an adult...!

(Source: Renta!)