HORSES
Clydesdales are a subset of horses
Geldings are a subset of horses
Mustangs are a subset of horses
Shetland ponies are a subset of horses
SeaHorses are not a subset of horses.
To argue that they are, requires emptying the word horse of all meaning first.
And possibly - nastily - casting aspersions on whether Shetland ponies are really horses, being so small, so unlike horses, better suited to being reclassified as large dogs perhaps.
Having deliberately, and artificially, brought the horse membership of Shetland ponies into doubt, seahorses feign outrage at this apparent injustice (created and maintained solely by them alone) and demand re-entry of the poor Shetland ponies into the horrid exclusionary parent horse group. (From which they were never actually excluded)
Accompanied by the seahorses, washed in on a manufactured wave of false outrage and fear and confusion by all horses that somehow certain entirely valid members of the group have inexplicably been unfairly excluded.
The argument that women (of any colour, religion, age) are a subset of women is not offensive in and of itself, used for proper purpose.
So long as the parent group 'women' has clear criteria, all indisputable members of that parent group may be subdivided into subgroups if that subdivision serves a useful purpose.
What is offensive is the TRA argument that black women (they always choose black) are a subgroup of women whose membership of the group "woman" is in as much doubt, as disputable as men's membership.
They contrive and manufacture doubt over whether black women are women at all, so that they can wash themselves into the group women on the wake of people "accepting" black women "back" into a group from which their membership was never, ever in question.
Yeah, that's pretty damn offensive.