Very sad that you have been put in this position.
Assuming you like your job enough and feel it is achieving something, ie you want to go on working there, maybe a direct confrontation isn't the best option.
Not sure the hierarchies ie who agrees framework for the network etc., but maybe in straightforward lets all be adults about this, can you not raise the point that based on recent court cases employers are meant to provide equal support to both the belief that TWAW and the belief that women are biological females.
So on a professional level, and to be in line with equal ops. the first meeting should have been about arriving at a democratic decision as the whether the "women's" network was based on gender identity or on biological reality.
No one, whatever job title had the right to jump and say what it is.
If they dont respond or say well no body else is bothered, then you could say you feel unable to participate, one because management has prempted the network establishing its goals, and two because your genuine beliefs are being denied.
Of course you will have to work out for yourself whether you think that will impact on your work relationships.
But I dont think you should have to battle it out. But they should be aware that their behaviour is the same as actually excluding you.
And for all you know if you challenge the pre-emptive assumption about the network membership other women at work may feel able to speak up.
Mull it over, rather than feel you have to immediately say something.
Do you have any idea if anyone else at work feels the same as you?
Hope you have a good sleep and feel refreshed and able to decide what to do.