There's a column floating around by Miranda Yardley about why it's okay to organize women's spaces according to sex assigned at birth. I wrote the start of a rebuttal and didn't want it to go to waste.
I'd suggest reading Julia Serano's "excluded" on this matter. I have a copy I can lend you.
A quick aside before I begin. I get that she's trans, but I don't know what is up with Miranda Yardley. Imagine having one vocal gay person who believes that marriage should only be between a man and a woman. Then imagine religious conservative groups plastering that person's name and words all over their media.
Yardley has long been one well-off member of a minority that takes the side of a system that is making life difficult (and often shorter) for that minority, including those without her advantages in life. The champions of that system can then point to her as a token example of a minority that agrees with them. Look at a sizeable chunk of the people who cite her columns. Look at what else those people have to say about trans folk. It's not pretty.
In my experience, the use of "sex" (or rather 'sex as assigned at birth') as a rallying point for "gender-critical" feminist organizing only developed because people wanted a way to justify their exclusion of transgender women that squared with a superficial progressive rhetoric. They used to use "youth socialization" as the benchmark of authentic womanhood as they assumed that no trans women experience it. Now that trans women are coming out young and do in fact get to grow up being regarded as female, they are moving over to "sex" in the hopes that this time it will stick and people won't look at it too critically.
It can be demonstrated that "sex-based organizing" is only a rhetorical device as it does not focus on uteruses another traditionally female anatomy in its political concerns. Further, trans men (who should, in theory be welcome) are not admitted, while women of variant intersex/reproductive statuses are admitted (when in theory they should not be). Rather, sex-based organizing relies on politics are otherwise identical to hostile outmoded exclusive policies that preceded them. Moreover, the spaces that tout sex-based organizing are pretty much always rabidly transphobic. It's not a coincidence.
This model does not acknowledge intersectionality. It oversimplifies being assigned male at birth as a free ticket and paints all trans women as being highly privileged dudes who one day decided to shuck off one identity to go slum it in another less privileged one. It's true that some trans women had no issues prior to transition, had alpha male careers and then made the switch, yes. However (1) it has yet to be demonstrated that excluding there are tangible benefits to excluding these (previously privileged, now marginalized) people, and, more importantly, (2) *most trans women do not experience this.*
I can tell you, most trans women can tell you, and any transgender-related quality-of-life statistic you want to check can tell you that if you are presenting as a woman, it is MUCH safer in this society to be perceived as CAFAB (Coercively Assigned Female at Birth - "female sex"). I go out of my way to get people to assume that I am CAFAB because I would not have been able to get my training, travel, be able to do my job, or even be safe on the street otherwise.
But if that does not satisfy the author, I can put it another way: We know that the majority of trans women (especially those who are visibly assigned male at birth, and especially those experiencing other forms of oppression such as immigration status, race, and ability) experience incredibly high rates of unemployment, violence, incarceration and mortality. Given this, is it just to go out of one's way to build a theoretical framework to exclude them from community and rights-based organizing?
Also to keep in mind: the exclusion of trans women from feminist (and lesbian) organizing operates pretty much in tandem with the exclusion of bisexuals, femmes and sex-workers. The logic in all cases is that one's personal gender behaviour (in partners, gender expression, identity, and work) must follow the party line. That ain't good feminism in my book.