So here's an explanation on the motivations of the mc in case anyone is still a lil confused:
Mc has been trying to project a false image to the public that he was "special" and "different", from everyone else. (by pretending to be a cultured, book-smart guy, and by pretending he's worshipping his crush for completely pure reasons, and not for normal horny teenager reasons, which, we saw was untrue bc of the whole bloomer-snatching incident, and the fact he confessed to not understanding his supposed "favorite" book.)
On the inside, he deeply fears being "normal" just like everyone else. maybe he feels that his life is a meaningless existence if he's just average in every way. Nakamura also has a similar worldview. she is sick-to-death of the way everyone around her mindlessly goes to school, follow the rules, acts politely, even fall in love just like everyone else does. To her, is all so painfully average and meaningless. She looks down on normalcy and anyone who conforms to society's expectations.
So, when nakamura got involved in mcs life and slowly eggs him on to do more and more crazy, abnormal things, he got hooked, they both did. And mc became more desperate to keep up his false image by doing everything Nakamura tells him to. This is why he repeatedly chose Nakamura over Saeki, no matter how head-over-heels saeki is for him. He prob feels that by continuing to date Saeki (and eventually going on to having a physically intimate relationship with her) would just prove that he is indeed, just an average teen boy, which again, is his greatest fear: normalcy.
Well... that's my take on it so far. Hope this helps anyone who needed it. Feel free to correct me on any mistakes or add anything I might've missed.