hi there! tysm, i really really love this question!! i think the answer is somewhat complicated (assuming that you mean this in the context of "they confess to mttw and he says that he's aro"):

bin: i will start by admitting that platonic sungseok is my favourite iteration of them and as an aro person, i believe their friendship to be something more beautiful than romance. that being said, i think if bin was rejected on these grounds he'd actually find peace in it. i think bin understands mttw very intimately and gleaning this insight would actually soothe any possible insecurities about not being enough or not being capable of being loved. i also think that Knowing that mttw likes their usual affections would comfort bin bc then he's not overstepping boundaries. i think he'd quietly struggle with his own feelings & try to lose them quicker as to not overstep mttw's boundaries, but i think this rejection would bring them closer.

(in a non-confession context where bin's feelings are always platonic towards mttw, i think he'd be deeply worried abt mttw being lonely or lacking in companionship. and i do thinkt that mttbin have a queerplatonic life bond so. take that as you will)

tael: with tr i always consider queerness & identity & so i think that again, this being a matter of mttw's identity & not tr's personality would soothe some of the sting. but i think he'd be incredibly embarrassed for not knowing before, even tho mttw didn't tell him. u could extend this to tr feeling "tricked" for more angst if you want. i also think that tr would require a much longer period of distance with mttw before returning to being friends as normal in order for him to like... ground himself. any sort of residual romantic affection would be followed up with a train of thought like "you idiot, you know that he doesn't mean it like that, stop being delusional". but all of this hinges on mttw explaining his identity to tr, if he doesn't and he's just vague about "not having feelings" then this is a FAR more violent and nasty fallout for them both.

geon: undoubtedly he'd have the hardest time, especially if we include the "mttw is gw's first love" narrative which i love so much. bc mttw is the first person that gw has ever felt this Strongly before, and the pattern of their interactions is very flirty and even romantic, and gw is smart so he'd see the patterns and assume that mttw likes him back. and geon really tries to be strong in the face of rejection, so he'd act like he's unaffected in front of mttw but he would spiral into self-doubt.

i don't think gw would doubt mttw's identity as being aromantic or even think that mttw could be changed, but simultaneously, gw would think that his feelings were rejected bc there's something wrong with him. these sound contradictory but it's the idea of like, rational understanding vs highly emotional and personal belief. it's a very childish, self-centered reasoning that i can't fully explain.

gw would need so much guidance from other people and so much reassurance before even approaching a point of being okay, and i could see an explosive fall out in the case of him not seeking any of that stuff out.