Been looking for this for like 2 years and can't find it…
So basically there's a childhood friend group of 2 guys and 1 girl; the MC is the lame friend (maybe fat?) while the other guy is handsome/popular, and the cool guy and the girl start dating each other. MC invites the girl to come over and study or something, then ends up forcing her to have sex. Later on, he tells the girl he wants to apologize to her and invites her over again, only to yet again force her. At the very end, the girl is like “don't be so violent next time”.
Hopefully this is specific enough. Let me know if there's any question. Thank you in advance!