Look.
Genderism is fundamentally sexist. There is no getting away from that.
Firstly, because the belief that the thing that makes men and women different is mental rather than physical requires that you believe some ways of thinking and feeling are only correct for "women" and others are only correct for "men".
And secondly, because it requires that the female half (and indeed the male half) of humanity cannot have language that refers to us and to us alone simply for the commonality of our sex.
And this even though being female has had serious consequences, physical, social and economic, that still affect how we experience ourselves within society today.
Which is crazy when you think about it! I mean, we have words to differentiate between various shades of ginger hair, but apparently not something as significant as the basis on which we have been abused, belittled, marginalised, even at times been legally literal propery of men!
Now sure, you can hide the sexism by pretending that "men" and "women" don't refer to sex snd claiming something that impacts on the female (or indeed male) half of humanity can't therefore be sexist because it isn't hurting "women" (or men) unfairly, but that just moves words around. It doesn't change which people feel the impact.
That is what people are reacting to.
Not some irrational desire to hurt trans people by pointing out that their gender does not override their sex, but the need to protect the reality of sex in law, society and culture because sex has consequences.
It isn't fair to just bundle female people legally, socially or culturally in with whatever it is mentally that trans women feel makes them a "woman" (or assume we are mentally outside whatever it is that trans men feel makes them a "man") without bothering to find out how we see ourselves or to consider what changing the basis on which we exist legally, socially and culturally means for us.
And really, all this can be solved in an instant by simply recognising that if sex and gender are different, as trans people themselves clearly prove they are, they should simply just have different names. Leave "man" and "woman", "boy" and "girl" to refer simply to sex and coin new terms, new concepts, to capture and understand and support this thing that is "Gender".
And not because that's the "right" meaning (words are just words) but simply because that was the meaning when those words were used to write our laws and histories and stories. It's the meaning those words had when Feminists came together to understand our shared experiences and the social structures that created them, so it's the most practical way forward. Otherwise we'd have to rewrite all those things to clarify they do and always did refer to the female half of humanity not sinply those who feel a commonality of a certain gender.
So please. Spare me this superficial idea that this is all about being nasty to transwomen. This isn't about them at all really. It's about the right of women to exist in our own right, retain our own language that links us to each other and to our history and our rights, and to ne allowed to describe our own experiences and needs and self knowledge entirely outside of a man's frankly irrelevant image of what he thinks we are.
No one is saying trans people can't be who they are. We are just saying don't use the language of sex to name it because that has consequences far bigger than trans people's feelings, however genuine they may be.