this is so funny because it poses it like a fully conscious choice, rather than amity not being aware that karlach had non-platonic feelings for her -- and was really horny and touch-starved, sure, but amity didn't think that had all that much to do with her, personally, until the exchange that went like --
karlach: i know i had designs on you, but i'm glad you found a little happiness with someone who could actually give it to you amity: aw, thanks! you're sweet :) karlach: yeah, yeah, i'm an angel, giving away my one true love to a bloodthirsty toff amity: yeah -- HUH???? ONE TRUE LOVE?? HUH??????
it FULLY blindsides her and leaves her reeling -- even more than she's already been! and she was already blindsided, just in terms of astarion. like, the way things have been going with him have been a nice surprise, but a surprise nonetheless! amity set out with the intention of making friends out of as many of the companions as possible, and nothing more than that. she certainly didn't anticipate any romantic overtures from anyone, let alone more than one person.
ever since the first time astarion propositioned her, she's gone on thinking 'alright, allies -- maybe friends? -- who have sex. people do this all the time. i can do this. just don't make something out of nothing and don't start getting feelings where there shouldn't be any. he doesn't want that, and you already know the unrequited thing doesn't feel great, don't do that again. you're on different paths anyway -- maybe after we all kill cazador and get rid of the brain worms, we can meet for tea sometimes, but he probably won't want much more to do with you than that, so don't read into this and it'll be fine!'
so the recent conversation with astarion means that she's only now allowing herself to think that she can read into this At Least A Little, and she hasn't even taken the time to figure out what exactly her own feelings are yet. she's wrapped up in unwrapping that line of thought when she talks to karlach, and afterwards she's a bit too stunned to think about anything, so instead of the confident, 'haha yeah i broke up with her, i'm very certain about you' vibe i think it'd be more like
astarion: so you really ended things with karlach -- amity: DID YOU KNOW KARLACH IS IN LOVE WITH ME astarion: ...did you... not? amity: NO!!!!
the rest of the conversation can continue pretty much as scripted, but first she has to spend a few moments despairing with her head in her hands over having inadvertently broken someone's heart -- in an obvious enough way that apparently the whole camp knows it, too

















