Ore no Doutei ga Suterannee!

Shinji Ryunosuke is a hot-headed troublemaker but a popular delinquent with many girls. However, he's actually bad with girls and is a virgin, which is a source of insecurity for him.

One day, in an empty classroom at school, he accidentally witnesses Aoi Saku, a quiet classmate from the next class, having sex with an older male student. His body reacts to the erotic sight of a classmate he barely knows, and he falls into self-loathing. But the next day, Saku himself unexpectedly appears before him!?
[via chill chill]