[quote DPotter]@PurgatoryOfPotholes
Because I used to be "Daniella" and used to insist that female humans could be men and male humans could be women
Out of interest - what changed your mind ?[/quote]
Well, there were lots of little steps.
Some of the big things were:
I started lurking on mumsnet at 21 and started spending a good portion of my online life here. So, initially, I was obviously saddened and disappointed by other mumsnetters' transphobia, and their refusal to understand that they were misusing the word "woman". 
But carrying on posting here broke down my own internalised misogyny about being Not Like Other Girls as I realised so many of us feel like that. Also, gender identity theory relies on the idea that all women over 40 who express the slightest doubt concerning it are brain-dead bigots.
The amount of cognitive dissonance needed to post on MN, believing that, when at the same time I hugely value the same women's advice on everything from employment law, relationships, how tohow to handle awkward interpersonal interactions over parking seek advice from older women on everything from know also meant that I