I know it's not my thread but I can answer that one, Aridane.
It's currently the most important issue because if we can't define the word "woman" in terms of the law or policy, then we're in line to lose all the protections under the Equality Act. And in lots of other contexts.
As an example, if single sex spaces turn into spaces for "anyone who feels like a woman or says they do" then we've lost those spaces. They become unisex. Women fought for refuges, for rape crisis centres, even for public loos. Losing them is massively retrograde.
The aggressive trans lobby are trying to frame it that 12 y.o. girls who don't want to see penis in their changing room are bigots. So, yes, it's a big deal.