People who ignore scientific fact and claim that there are more than two sexes and that you can switch between, and that transwomen are women are 'othering' themselves, Darker.
They put themselves in the same category as people who believe in astrology or that the earth is flat or that 5g fries your brain or whatever. Some of these beliefs are less harmful than others, but they all have the shared feature of being non-factual.
So anybody who claims that human sex is a spectrum not binary, and you can change from one sex to another is putting themselves in the category of 'OTHER: DOES NOT ACCEPT SCIENTIFIC FACT'
I think most of us would have no problem agreeing with you, Darker, that 'gender' should not conform to a binary definition. It should be possible for little boys and little girls to grown up doing whatever they damn well want, and not be pushed into pink/blue/princess/pirate stereotypes.
In particular, it should never ever be suggested to children that if they feel they don't correspond to a stereotyped 'gender binary', then they must be the problem - they are in the wrong body, their gender identity does not correspond to their biological sex, they have to change themselves - psychically, emotionally, physically, pharmaceutically, surgically.
Gender critical feminism says fuck that - it's the 'gender binary' stereotypes that have to change, not the children.
Anybody who believes the opposite is 'other'.