The thing is, the ONLY reason women have our own specific rights, spaces and services is because we have female bodies.
Like all mammals, our bodies are, in the vast majority, smaller, slower and less strong than the males of our species. Like all mammals, it’s the human females who gestate and feed the offspring, and it’s our bodies that suffer the discomfort, risks and consequences associated with being the reproductive class.
People who don’t have female bodies will receive absolutely no practical benefits by accessing women’s legal rights, but if we can’t define women as ‘adult human female’ it will significantly weaken our legal protections. The ones designed specifically for our bodies.
That might not sound like such a big deal in the privileged west but human rights laws are international and if we can’t retain our protections in the first world, the women of the developing world will never get them.
We need political representation by members of our own sex class in order to protect those rights.
I totally support trans people’s identities being similarly, but separately protected in law, but lumping transwomen in with women, despite having completely different social, legal and medical needs, will leave us all fucked in the long term.