The Process of Impure Love

Overview

"Is this enough for you to promise not to tell anybody what you saw...?" One evening, ordinary and completely straight college student Serizawa comes across two men kissing in a dark alleyway. As he attempts to walk away unnoticed, he recognizes one of the men as professor Utsumi; a lecturer at his college. Serizawa finds himself becoming interested in the love life of Utsumi, who is well-known for his iron mask and expressionless demeanor. In exchange for keeping Utsumi's secret, Serizawa invites him out for drinks. Later that night, Serizawa misses his last train home, and ends up dozing off at Utsumi's house. He wakes up, and, to his surprise, Utsumi is going down on him! A bittersweet, impure love story between a student and his plain yet erotic professor. (Source: Renta!)

Source: AniList