The group I was in when I had breast cancer treatment was only for women under 45. It's rarer in younger women, and their needs and concerns are different from older women. Most of us either had kids, or wouldn't be able to have kids due to treatment (not all, obviously some were childless by choice).
The honesty was overwhelming and really comforting. Bluntly, losing breasts is a big deal to women. That's not the same concern for a male patient. The hormonal implications are likely to be different, too, in a wide range of ways. And chemo affects all parts of your body, which can be grim on the dignity front. A lot of us talked about symptoms we'd never have done outside that group in a million years. Relationships, too. Self image. All sorts of ways in which being vulnerable and rawly honest was the norm.
I'd not have joined, far less posted in, a mixed sex group. And I completely sympathise with men who feel the same way, and want their own. I'm really glad that they have that.
Lived experience matters. Solidarity matters.
And what on earth has it to do with trans? Why is this man even raising that? It's just to do with the need for suitable single sex provision. I'm really glad he has that, or mixed sex groups, available to him. I'm really glad older women have support groups available to them, too. I wanted one that understood what it is like to be a woman having breast cancer young. My group gave me that. By definition, if it took older members, or men, it couldn't.
It's okay to have places that meet the needs of specific demographics. Not everything's for everyone.