[All these points apply to sex and to gender, so for ease of reading, I'll just discuss gender]
Gender-exclusive groups are common in many societies, such as men-only and women-only social clubs and casual activity groups like a men's bowling group or a women's reading circle.
Sometimes this is de-facto, but sometimes this is enforced by rules or expectations, treating the club as a safe space for airing issues people have with other genders, or avoiding perceived problems with other genders.
I came across this old comment in a garbage subreddit by accident when researching. The topic is Men's Sheds:
"Here's the thing. No reasonable person has an issue with women having their own women's activity groups. The annoying part is that whenever men try to do something similar, that's a problem. Women either want them banished or demand entry, EVERY time."
I think their claim is nonsense, grossly exaggerated at best. I also know of many counterexamples of men trying to get into women-only groups (as an extreme case, the Ladies Lounge of the Mona art gallery in Australia was taken to court for sex discrimination, with the creator claiming they would circumvent the ruling by installing a toilet). But nonetheless, I can understand why they feel this way, patriarchal social relations change how most people see men-exclusive spaces vs. women-exclusive spaces.
But my response to their claim is that, I am reasonable and I do have an issue with any group setting up places which discriminate based on gender. These safe places can form as a legitimate rudimentary form of protection, yes, but they maintain and often even promote sexism, and should all be challenged and turned into something better which serves the same purpose.
Of course, I'm limited by my own experiences and perspective, so I'd love to hear your opinions on the topic.
Bonus video: "Why Do Conservative Shows All Look the Same? | Renegade Cut" - a discussion about fake man-caves and sexism.
Sure, of course they are.
I'll even go so far as to say that even more fine grained groups are okay. What becomes a problem is when every group excludes people that really shouldn't be.
You get a chess club, why the fuck can't a woman join? Right? Calling it a men's club is just exclusionary for no purpose. Even the girl/boy Scout divide was pointless in any real sense, and was a missed opportunity for those scouts to have guidance on how a scout is supposed to treat others.
Hell, when it comes right down to it, even a specific cis organization is fine, just the way trans specific ones are. The problem, again, is when a club is exclusionary just for the sake of it.
We all have aspects of our lives that aren't shared by people with other genders and/or types of genitals. There's struggles and discrete experiences that a trans man can have that I never will, and vice versa.
But, again, once it ceases to be about that kind of specificity, it starts being bigotry in disguise and needs to fuck right off. Ain't no good reason women shouldn't be allowed into things like community action groups. A gender division there is just pointless and stupid. If they also exclude trans men, it's as bad (maybe even worse).
Hell, the masons are full of shit in that regard. Fraternal orders are hypothetically okay, but since when have the masons actually been about men sharing the unique aspects of life that men share? It's just exclusionary bullshit (and I've seen it from the inside, so I know it's utter bullshit). They're the best example of how not to be a gender based organization.
I'm not saying that men shouldn't be able to gather and just hang out. We should, as should women. There really is a different vibe, and there's no way around that. But once you start organizing that on a bigger scale, you have a different threshold to meet.
Since, historically, most of the men's organizations not only excluded women, but actively served to continue oppression of women, being a de facto patriarchal enforcement group, those groups get the worst attention. They weren't really men's groups, they were power control groups that men only could use to gain, maintain, and exploit control. That's why there's pushback on them, not the fact that they were/are gendered.