Dude would actually have a chance to get his head on straight if he just realized he's not the only perv in the world (and probably either the least or second-least worst in this story). The basis of his self-loathing-to-masochism slide is his belief that his sexuality with our without its kinks is uniquely vile and he hasn't had anyone around him to clue him in.
I chalk it up to the mangaka's drawing technique. But hey, they can create compelling characters so the drawing isn't really an issue for me most of the time.
So this girl is meta-NTR’ing MC, the same MC who is irl-NTR’ing her with three bullies, one of whom is being NTR’ed by the main bully, and that main bully is being NTR’ed by another bully in her group. Do I have this right?