Where did that guy go wrong? Now he's got everyone against him and he still doesn't get it...
The short version is that he was taught to be like this, going by the previous chapters probably mostly by his mom, who is a serious piece of shit.
Conservative Confucian values put a lot of importance on gender roles and having male heirs. It's real bad. People who don't have male heirs are devalued and looked down on. This has historically led to systemic misogyny and an obsession with having at least one male child and a tendency to devalue female children, sometimes to the point of abandoning or murdering them. The story referenced this earlier on, I think with Xiaoen's Grandmother, (dickhead's mom, which tells you where he learned this bullshit) who wanted to throw her out when she was a baby.
It didn't used to be this bad, having a girl used to just mean you kept trying and tried to find someone to marry them off to, (which still isn't great but it's better than murder,) but this devaluing of female children is compounded by China's "one child" policy, which, in a desperate attempt to curb overpopulation, limits families to having one child each.
If the one kid you're allowed to have is a girl, the only way you get to try again is if you get rid of the girl somehow. Pawn her off on another family that doesn't have a child, or, y'know, accidents happen.