Gender exclusive groups are OK when there is a legitimate reason. Unfortunately it just so happens that women-exclusive groups have a legitimate reason very often, which is usually “I don’t want to be hit on in every activity I do”.
Why are there women only career events? Because many women experience going to “normal” career events, have nice conversations, thinking they made a good business connection just to be asked out on a date and ghosted when they decline. They don’t get the same benefits out of “normal” events as men do.
Why are there women only gyms? Because women want to do sports without being hit on regularly.
Now you could say “Well, but that’s a problem of some men not sticking to the rules. Just enforce the rules.” But the problem is, the rules aren’t being enforced, women aren’t taken seriously or just told to suck it up, that’s part of life. You’re in a public space so it’s OK for a man to ask you out. To which the women’s reaction is: “Well, then I’d rather do X in a private space where there aren’t any men who could hit on me.”
As long as there are struggles that men face exclusively it’s totally ok to have men only groups. The problem:
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men do not face the problem of being put in uncomfortable situations by women almost anywhere they go, so they have less topics or activities where they feel like they need a men’s only group. For most topics/activities men can go to a mixed-gender group and have the same experience as they would in a male-only group. Women can’t.
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a lot of men’s groups do not form around “we want to address a typical male problem” but “we have prejudices about women being bad at x” or “we just hate women”.
And lastly historically the reason why women wanted to join male-only groups was because those groups were often used to make decisions and policies. Business is being made in golf clubs and was made in “gentlemen’s clubs”. Women wanting to join those wasn’t about playing golf. Sure, we can have a women’s club to play golf. It was about being left out of the informal decision making process, the deal making. In my personal experience women are more likely to discuss work matters at work with everybody and at any “women only” outing with colleagues work was hardly a topic. Whereas when it happened that men went drinking with “just the boys” the next day important decisions had been made and suddenly Mark was in charge of the new project. Just my personal experience and I’m not saying it can’t happen the other way around in female dominated fields.



I’m not talking about friend groups, just groups that are open to the public. Friend groups are OK in whatever constellation you wish.
Your choir has a good reason to be men only, since that creates a certain sound.
It gets tricky when the point of the group or club is something not related to gender. I don’t think an all-female board game club that is open to the public but only lets women join would be OK. Personally I think you can have your meetings for only people of your gender when you organize them only for yourself. But as soon as you do something publicly, you don’t get to say “everybody can come except group X” without a good reason.
That goes for men and women, I’m also not a fan of “xy only for girls” clubs without a good reason.