Samoa has recognized three genders for centuries. In the ancient Mediterranean world, eunuchs were their own gender.

It's indisputable that gender is a social construct.  That doesn't mean that individual people get to change or add to it on a whim.  Our construct of gender has for centuries been based on biological sex, and there is no good reason to change that.  What individual transgender people seem to want is to pass as the other gender with no complaints or negative comments.  They would still mostly agree that there are two genders.  But what the movement writ large wants is to add all new "non binary" categories to our social consensus, and it's dangerous to change a social consensus that quickly, for obvious reasons.  The actual motive is probably to prime us for a greater social upheaval later.

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