Yes for me being with only other women gives me firstly a feeling of security and safety, that I don’t have to feel on edge in the way I do around men. It’s not that all men are dangerous or that women can’t be aggressive, of course, but men are stronger and more likely to be aggressive, especially sexually. I am a sex abuse survivor (though I would also support any woman who felt this way for any reason, or none) and that “on edge” feeling is built into me. Being among women only is a lovely, warm, relaxing and reassuring feeling.
Secondly it is about what you can talk about. Being a woman/female is a physical reality with many body issues and experiences no male will ever know, and being able to chat about and share them and laugh about them is so supportive. But even just a normal, well-meaning man in a female group changes the dynamic because talking about these female experiences is embarrassing for both the women and him so it just stops. I had a group of female friends in the past who would meet up to do crafts and it was lovely, but then one started bringing her husband because they were joined at the hip and he was bored at home (or he was controlling, but that’s what she said). He just basically sat and watched and said nothing. But it wasn’t the same, the conversation dried up and it wasn’t fun any more. Of course women don’t just talk about periods and childbirth, at all, but it’s that feeling that you can say anything and other women will get it.
add transwomen into the mix and it’s different again because not only does the person have no genuine experience of being female, but it’s considered offensive to remind them of that, so now you risk being not just embarrassing, but offensive and as has been reported, could even be reported and chucked out (depending on the nature of the group). And for some TW, they want to join women’s groups for validation and sexual turn on and so will seek them out, and try to harass and punish any group that doesn’t want to allow them. A truly respectful male person, whether trans or not, would understand that women having an all-woman group share something that they don’t, and would respect that. (And some transwomen do respect that.) so you’re left with the ones who don’t have any respect for women or their feelings or needs. They’re the ones who want to be there.
Of course if you do want your group to be inclusive of any male who says he’s a woman that’s a valid choice too but it’s not a women’s group. Even if you thing transwomen really are women, there’s no way to tell who’s a transwoman and who’s just a man saying he’s a woman for his own motives, which could be sexual or predatory. You’re opening your group up to any male who wants to be there. And that’s why many women will stop going. I no longer bother with any woman-only crafts, adventures, clubs, etc at all because when you look closely it always says “women and anyone who identifies as a woman’ so I know I can’t expect to have that cosy reassured feeling, and not only that, but if a male is there, I have to suck up to him and fake seeing him as a woman, or be punished. No thanks.