Paulo and Sue, and Sexism
Paulo is something of a lady's man, a flirt who hits on girls freely and attempts to charm them. He flirts with basically every girl that moves in middle school.
But he's never makes a pass at Sue, even though they were on pretty amicable terms all the way through freshman year and the beginning of sophomore year.
When pressed on his opinion of her as a romantic candidate, Paulo describes her as "kinda cute", but it seems really half-hearted, like he knows there's objectively nothing wrong with her appearance but he just isn't into her. Her personality's part of it, but that didn't stopped him from pursuing Katie, who he described as "selfish and bratty". Sue just isn't his type. Which is fine! There's nothing wrong with not being attracted to Sue, and she probably wouldn't appreciate his romantic advances.
But Paulo clearly treats Sue differently than her female peers. He's rude to her, casually disrespects her, and puts her down completely unprovoked in a way he just doesn't with the girls he likes, starting back long before his relationship with her truly became contentious:
So Paulo is rude to Sue and doesn't respect her. He's also a jerk to Abbey and Mike early on. But it can't be ignored that Paulo does not have any female peers he both respects and isn't attracted to. We can't extricate his treatment of Sue from his sexism:
Paulo says this to get a rise out of Mike, but it's very much at Sue's expense. He's reducing her to a sex object, with the implication that the only reason Mike would be inclined to do the play with her is for sexual reasons; Sue as a person or an artist has no merit. Paulo personally doesn't find her attractive, but he's still is reducing her to a sexual entity because he fundamentally doesn't respect women:
Paulo explicitly frames himself as valuing women's bodies and not their personalities, characterizing women as conquests and objects to be won by men. This is deeply gross and sexist. Even if we've seen him care about women in genuine ways, this is how he talks about them. He's broadcasting a worldview that values women based on how sexually desirable he finds them first and will only maybe consider them as people second. And it's not like these examples are all ancient history; two of these are from within the past year and reinforce that he still believes those earlier, nastier, explicitly stated positions.
And 4/5 of these examples happen in front of Sue. She's aware that this is how he thinks about women. And Paulo also has specifically attacked her body, the only metric he claims to value in women:
Sue is failing to live up to her ideals here; she thinks fatphobia is bad and is trying to discourage mocking people for being overweight, but she is still a teenage girl who is insecure about her own body. Paulo takes the opportunity to insult Sue's body in a scenario that will render her a hypocrite for being insulted by his insult. He's doubling down on the notion that it's okay to make fun of and not respect girls he doesn't think are attractive.
Paulo only has been shown to respect women he's attracted to, and he laughs at the idea of being attracted to Sue. That's part of why she gets so furious in response to his laughter.
Of course all of this is is going to make Sue think he's really sexist! Sue is overtly unimpressed by Paulo's objectification of women and is someone who thinks about gender politics.
And if you're Sue and Paulo has communicated that he only is willing to treat women well if he's attracted to them, the implication winds up being that he's never going to respect you, no matter what you do or say. But Sue so deeply wants to be respected. She can't debate her way into being respected with someone who's deeply sexist. Sue's not going to be vulnerable and ask for respect when Paulo's already given her an answer about how he sees her and women as a whole.
I think Sue commits so heavily to the position of "Paulo's and idiot and therefore undeserving of respect" in part because of this. If Paulo's never going to respect her, then she's never going to have to respect Paulo right back. She doesn't want to see qualities she respects in Paulo when he's positioned himself to never respect her.
The lack of respect obviously has exacerbated Sue's insecurities, and there's more that goes into their dynamic than this, but the sexist elements are palpable. Paulo may have grown a lot, and his treatment of women has improved significantly, but he still doesn't have any female peers he both respects and isn't attracted to and has started making up sexual scenarios to degrade, humiliate, or provoke women- including Sue.
Sue's ego taking a hit in a competition isn't going to fix her issues with Paulo; it's just going to make her feel worse about how Paulo doesn't respect her. Paulo needs to extend respect to Sue to actually break this paradigm. I believe he's capable of it; he just has to prove it.