Fujunai Process不純愛プロセス ; The process of unplatonic love

On his way home from a drinking party, ordinary and straight university student Serizawa comes across two men kissing in a dark alley. And one of them is none other than Utsumi-sensei, the boring, expressionless professor! One thing leads to another, and Serizawa finds himself becoming interested in Utsumi-sensei, whose looks and personality don't match