A lot of people also defend the first kiss also as one of those old movie tropes: "Sure, he kissed her without her consent, but it's fiiiine because they ended up together." Usually these violations include a "yeah, but" excuse, wherein they have the female character like or be okay with what happened to them.
But 1) They didn't even have the decency to include her liking it as a "yeah, but" and instead had her be visibly upset after- making the wrongness of it blatantly obvious and uncomfortable (but then doing nothing with that and retroactively acting like that was a symptom of her being "confused about her feelings" blech), and 2) Just because the writers force a character to be okay with something that happened to them, does not make what happened to them okay.
It's also just completely unromantic. Because the way it's set up isn't some grand buildup of feelings between them culminating in a gesture. It's Aang imposing his crush onto her without any conversation or warning or consideration of her feelings. It gets even worse when he uses his forced kiss as essentially his claim on her, his reason he's mad that she isn't just with him now in EIP when she never got a choice in the matter.
And when she says she's confused he tries to force it again when her guard is down and she's obviously unable to react. Which is a symptom of the idea that women just play games and secretly want it and to keep pressuring them, which is made worse by the fact that they reward this belief by having her end up with him anyway and never show his entitlement to her as wrong. It literally backs up the idea that all her negative reactions to his advances, her general disinterest, and her avoidant body language don't need to be considered because she, again, just secretly wanted him all along or wasn't important because she'd learn to love him.
And the idea that Aang is too good to be abusive to the point that the idea is preposterous mirrors so well the whole "I know my boy, he wouldn't do that" denial/excuse used to shut down women talking about abuse. You don't have to be evil to be abusive in a relationship. No one is mad because they think he's evil; he just should have had to learn from his poor behavior. Every abusive person I know means well and is thought of by everyone who knows them as a good person. The 'nicest' person I know around others, considered to be the perfect prince any girl would be grateful to be with, was an actual villain in his last relationship. Not out of ill intent, but just by not considering the shitty positions he was putting his gf in and only worrying about what he wanted (sound familiar?).
"But he's only 12! He's not allowed to make mistakes?"
-> Then he's not ready for a relationship and shouldn't be in one. This is not a punishment because no one is owed a relationship with another person.
-> Then he's definitely old enough to know better than to put hands on people, and definitely old enough to learn his lesson about respecting others' boundaries- unless you think it's harsher for boys to hear stern words than girls to be violated.
-> It's not a 'mistake' if it is never acknowledged as one and continues to persist. It's just a character flaw. One people are allowed to criticize when it goes unresolved.
-> We have no reason to believe he'd grow out of this behavior because it was never addressed and was in fact rewarded, and Katara has no obligation to put up with this behavior before it is fixed, or even after it has been fixed. She should not be expected to be with someone on the basis that in the future they might rise to the bare minimum standard of respect for his significant other.
"You're acting like he's worse than Zuko (when he was her enemy)"
-> This is an incel talking point. 'He's not as bad as xyz so he deserves to get the girl/be in a relationship.' So, because he hasn't literally attacked her (the Avatar state putting her life in way more danger than Zuko incapacitating her in a duel notwithstanding), he's automatically a good partner? His violations are above criticism because he could be worse?
-> Zuko was on the opposite side of the war, it's expected he'd fight her and since he owes her no loyalty, it isn't a betrayal (until the Catacombs, which has a whole arc to mend that personal betrayal). He also is shown by the show to be wrong, is punished, and makes up for everything he did. Aang is her friend and actively owes her respect for all she's done for him and he still violates and does not respect her or her feelings and never is shown to be wrong for it. He never has to overcome it. He is instead rewarded for being a bad actor (treats Katara horribly: gets rewarded with Katara as a love interest; abandons his friends and runs away at the end of the world: gets rewarded with a magical fix to his dilemma). So those negative behaviors still exist within his character to be criticized. Zuko's aren't there anymore so there's no point.
-> Yes, her friend should be expected to treat her better than someone pitted against her in a war. And he still treats her like his prize he's entitled to and tired of waiting for.