why do you keep changing the topic of discussion? first its "x is bullying" now it's "mc is annoying" you don't even know what happens next because that is when it ends, also, why should he have to marry her? it's his life too, and marrying someone just because they don't want to be in an arranged marriage is just mindfuck levels of weird, are you projecting too hard on the mc? Also the mc said he "WAS IN LOVE" with her, WAS, so it's in the past, don't mean he should still have to like her, your thinking about this is too simple, do you also jump on every occasion some woman shows slight interest in you or needs your help with something because you want pussy? if yes you should re-evaluate your life.
He HAS to do nothing, if niichan writes it as it being your choice then it's your choice, if niichan decides to have him decline for now it's his choice as dev, this is a story, not a choose your own adventure game, although you have to some extend some choices you can make it's only as varied as niichan makes it, if you don't like the story, you don't have to play this visual novel, niichan said the same thing