There’s this one trap manga where a guy goes on a subway with his girlfriend but to protect her from perverts, he lets himself be the one to get groped by perverts on the subway many times, until one the groper finds the boyfriend then blackmails and fucks him im pretty sure. Thank you
Handsome Boyfriend Series Volume 1. – Her Straight Boyfriend’s Descent Into Gay Pleasure
this one?
https://nhentai.net/g/164711/
I think it’s probably what you’re searching for. This is chapter 1. It has two more.