It's not rocket science.
You try to draw analogies with racism but you ignore the fundamental difference - black people don't want everyone to pretend they are white as if white were something meaningfully different or better, they just want society to stop unfairly limiting what black people can be.
Men wearing dresses, challenging sexist constructs of man and women, proudly appropriating feminity while saying "all this and I'm a still a man, worth as much as any other man, and must be accepted as one" would be progressive and open.
Men wearing dresses, challenging sexist constructs of man and women, proudly appropriating feminity and saying "all this means I'm actually a woman and must be treated like one" is regressive and sexist.
You don't understand because you have let people brainwash you that somehow something in trans women's minds makes them "women". You have accepted the sexist idea that being a woman is about how you think not just being part of the half of humanity that is female in all our glorious diversity.
So you start from the unjustified assumption that trans women somehow are women, but have never actually asked yourself how that can be, what does it really mean, and what it means for the half of humanity that is female to be redefined as a personality trait.
Because let's be clear, calling trans women women isn't opening up womanhood, it is reducing it. It is taking female-specific voices, experiences and perspectives out of womanhood and boxing us into being only things that can also be true of male people. And that might be good for male people, but it's very bad for us.
So you miss the battle you should be fighting and instead find yourself fighting for gender stereotypes, sexism and the suppression of female voices, history and rights.
I realise if you have a "trans" child you have to erect psychological barriers to thinking about this properly.... but try anyway. Recognise those thoughts that your mind is telling you to stay away from, be brave and face them head on, and most of all let yourself be honest!