Haven't read all the thread so apologies if I'm repeating things that have already been said.
I think part of the reason why people don't understand why radical feminists wish to meet together in women only (ie FAAB) spaces, is because they think that radical feminism is about fighting inequality.
But that isn't really what radical feminism is about - it is about fighting against male supremacy, female oppression and male violence. It is about fighting against the gender binary hierarchy and systematic and institutionalised oppression that women suffer because they have wombs.
Women, as a group, are not oppressed because we have vaginas and breasts (although those body parts are used to oppress us), we are oppressed because we are the group that carry and give birth to children.
And this is where trans and radical feminists are poles apart. Trans people are discriminated against because they are trans. Women are discriminated against because we have wombs, ovaries, etc.
Therefore the discrimination that trans people have to deal with (which I agree is far from negligible) is NOTHING like the historical, systematic, institutionalised oppression that women experience and which, indeed, forms significant parts of our group identity and socialization.
It is perfectly legitimate for radical feminists (or indeed any other group of women) to want to gather together and share common experiences and find strength, comfort and support in numbers.
There is no need for them to include every group that is marginalised or the victim of patriarchal inequality.
Radical feminists do not wish to deny transpeople their get togethers and mutual support - we just ask to be allowed the same.