I don't get this conflict it at all. Why is she suddenly decided that she is just sex toy for him? There wasn't a single clue to lead her to that conclusion, besides maybe his over the top sex-drive, but it's not like she was being neglected or something. He was considerate of her and gave her more than enough attention not only in terms of sex.
The reason for drama is pretty good itself, but realization is kinda meh and feels too forced
Read back couple chapters or again, the muscle brain basically knows about sex from porn and half assed info from mom, it's nothing but "slam dunk and bam" rough sex so naturally it was always one sided as she mentioned it to his surprise.
She felt he is using her to vent his sexual urge and since even if they did it countless times now, didn't came even once (yikes) because he was the only one satisfied.
She was neglected.
He's motivation was always about sex, be it study or sports.
It's about teenagers discovery about relationships, them being cock blocked by public morale committee, shitty sex ed and whatnot.