*If it's unrelated to physical sex, then how come it's all people with the physical sex female who get that role applied?
Sex is the root, gender is the tool.*
I'm not saying gender is the root I don't think, I accept it is underpinned by sex, but as you say gender is the tool, gender is what justified women not being allowed to be pilots, and as gender expectations changed so did that assumption. Nothing changed about people's physical sex.
Women have been oppressed globally forever because we are the ones who have the babies, men want to be sure it's their baby, so various mechanisms are created to keep us controlled.
I think that's part of the story, but I lean more towards Selma James and Marxist feminism in that one of the main reasons for gender (and sexism) manifested in the way it did is because of capital's need for an army of unpaid reproductive and domestic workers. Western capitalism no longer needs that to the same extent and I think that supports a materialist basis for the breakdown, or weakening of gendered assumptions about what work we do.
I also think the desire for male control was not just about having babies but also about ensure women's sexual availability. That's why men wanted to own women and created very strict protocols amongst themselves about how male access to womens bodies should be organised. I think a lot of the second wavers were right when they said marriage was a form of socially enforced prostitution. That's changed to some degree, but now women's sexuality is commodified and exploited through porn, sex work etc. I think this is something that equally applies to trans women, as does the ever growing pressure to adhere to capitalist beauty standards. I wonder if this is why the generational split has emerged to some degree, there is clearly a common oppression and common struggle between trans and non trans women when it comes to sexual exploitation by men, and perhaps young women feel that oppression more acutely than the pressure to have babies and be housewife.