To return to the title of the thread, Why is anti-trans so important?
Re-stating the fact that it's not 'anti-trans' to be pro-women, I'd like to take a wider perspective on the transgender phenomenon.
Future historians will look back at this era and try to find explanations for how it was possible for an idea which has no scientific merit to be able, in a short space of time, to turn large sections of society upside down, mostly in 'Western' countries.
It is important to know how in a very short space of time, the scientific fact that human sex is binary and immutable was dismissed as hate speech, with such power that even scientists went along with the dismissal.
It is important to examine how school curricula abandoned fact for fiction about 'sex is a spectrum', and how - contrary to the promotion of body positivity which preceded it - children were informed that they may have been born 'in the wrong body' and offered drugs and surgery to 'fix' their wrong bodies.
It is important to understand how a media which believed itself to be free and responsible took to deliberately calling men women, even when the result was clearly nonsensical e.g. 'her penis'.
It is important to investigate how a tiny percentage of the population in such a short space of time came to wield such power in society that they could demand changes in the law, in language, in education, in medicine, and even demand the provision of separate facilities and services for themselves, at a cost entirely disproportionate to their number.
It is important to understand how people were portrayed as hate-filled, as phobic, and even as criminal, for resisting the wave of non-facts and compelled speech.
It is important to record the women driven out of their jobs, physically attacked, excluded from friendship groups, shunned by family members, for asserting what was common knowledge just a few years before.
There have been sudden waves of irrationality in past history - the witch hunts, the Dutch tulip mania, the South Sea Bubble - which historians have studied and attempted to contextualise and explain.
It's important to do the same with the current sudden, powerful and damaging wave of irrationality, i.e. trans ideology.