A common problem with this approach is that:
Trans women routinely show up to events and spaces that are classified as for women, and will be turned away at the door or kicked out if they don’t 100% pass.
Trans women will go to these spaces/events, and learn that the working definition of woman is ‘cis woman’, whether in rules for entry, or in how the space and the dialogue within operates. When trans womanhood isn’t considered to have valid experiences of womanhood that don’t also overlap with cis women’s, that absolutely excludes us. For instance, a lot of my cis lady friends feel that having had periods, and pregnancy are prominent experiences of their womanhood, and that’s valid enough, and it’s fair for discussion to reach those experiences. But if I’m not allowed to remark on how my body provided prominent experiences of womanhood as well while growing up? Then that’s going to send me a clear message that my womanhood’s not acceptable. And sadly, there’s not a lot of cis women out there willing to accept my body’s female, each and every bit of it, and that it can have played a part in my experiences of womanhood.
Trans women will gain access to these spaces/events only to be harassed and treated in hostile ways by those in attendance
Trans women will go to those spaces/events only to find trans men and transmasc nb folks in attendance, which 99.9% of the time means we’ll be treated as men/male, or more of men/male than the trans men/trans masc folks, that we’ll be treated as outsiders to womanhood rather than insiders in contrast to trans men and transmasc nb folks.
These “women’s” spaces/events can be named after things that trans women do not necessarily have, like vagina/vulva/uterus-based slang/terms, or XX chromosomes, or whatnot. This sends a clear message that those of us without those traits are not welcome, especially when not specifying trans woman-inclusion, since most folks who do this are essentialists that equate womanhood to those traits.
In short, there’s too much cissexism, so making it clear that trans women are welcome can send us a clear signal that our time is less likely to be wasted, that it’s more likely to work out, that those involved are more likely to have done some processing of cissexism and transmisogyny in order to declare themselves inclusive to us, etc. etc.
Ideally, just saying “women-only” or “women’s space” or whatnot would be sufficient. Sadly, in practice, that’s just…not how things go down. We are women, but most don’t accept that. Most people’s working definition of ‘woman’ is effectively ‘cis woman’, or people they perceive to be cis women.