You could look at it that way about Kurose and his behavior toward the ex, or look at it the way it was presented which was that she was homophobic and coyly insulting Shirotani. Her confidence means she’s stepping on Shirotani’s existence, not giving any regard for his presence thinking she can swoop in a steal his man and Kurose was not having any of her bullshit. I say it was just. Treat others how you want to be treated.
While she wasn't aware that they were together I get where you're coming from. But the fact is that he didn't address her homophobia or arrogance and her behaviour toward Shirotani. He only addressed her confidence and his kinks. Saying "I would have considered it (going out) ... if you would have acted scared before getting devoured" From what he said I assume he didn't reject her because of her behaviour bur rather because she didn't "act scared". (Obviously the fact that he'should in love with Shirotani contribute to why he didn't date her but in my opinion he should have told her off because of her behavior and not his kinks)
I honestly agree that Kurose-Kun shouldn't take any of her bullshit and she should be treated equally as harsh but maybe with a different context
I love this manga. It was absolutely brilliant. I would have liked to see more about Shirotani and his father. They used to be so close but the only flaw from my perspective in this manga was when Kurose-Kun insulted Shirorani's dad's ex not because of her bad personality but because she was confident when she asked him out instead of acting like the stereotypical shy, Virgin "I always look like I'm about to cry, pee myself and then cry again" type of girl. The rest was absolutely amazing and heartwarming.ヾ(❀╹◡╹)ﾉ~