I can’t remember the title, but everyone is Highschool age. Uke has unrequited feelings for seme, and offers to let seme try sleeping with him?. Seme decides to keep having girlfriends since sleeping with uke isn’t cheating.
Uke is hurt and feels like seme is using him, so he decides to break things off.
Another character offers to pretend to be his boyfriend.
Ultimately Seme loses mind and rapes uke in a library.
Has anyone seen something like this?

Found it! It’s Yabai Kimochi
Looks like I got the ending confused with another story.
https://www.mangaupdates.com/series.html?id=4304

Ending was Kimi no Kao ni Sasu Kage
https://www.mangaupdates.com/series.html?id=3212