I don't think this is necessarily the case. All the gyms I've been to have been majority men, like 80%+.
I think the "activities" part plays a role. I think a lot of men today struggle with socializing, especially with groups that have a good chunk of women. They feel like they don't belong and are acutely aware of how they can be perceived.
The gym, on the other hand, is a pretty isolated thing.
It probably says more about how men socialize tbh. And we know that fewer and fewer men are "having fun" in the dating world, which extends to antisocial behaviors like for example, not joining a run club because of perceived awkwardness.
A very attractive mild acquaintance was either bored or lonely or whatever and set up her own run club, one guy showed up. Without wishing to think like a PUA these are good odds.
Go into most gyms and it’s the opposite phenomenon. Men know that some types of physical activity (weight lifting) actually get them laid and others (I.e running) are the equivalent of what food does to get away from predators. The men who get good at that type of physical activity tend to be treated like the food is.
There are plenty of women who prefer guys with runners' physiques over guys with muscular physiques.
There was a dating website now-owned by Match Group that did a study years ago (pre-acquisition) that showed that college-educated women preferred guys with runners' physiques by a lot over guys with weight-lifters' physiques, and women without college degrees preferred muscular men by a lot over skinnier guys. In both cases, it wasn't even close. Unfortunately this study was taken down after they got acquired or I would link to it.
others (I.e running) are the equivalent of what food does to get away from predators
Running is how predators in nature catch most of their prey. Strength is not relevant until after they've caught their prey.
Women like men who look good. Men who lift weights usually look more attractive than someone who has a “runners” body. (Skinny and not muscular) Some runners will look jacked but that’s probably due to steroids and/or weightlifting. Running alone doesn’t give one bulging biceps or a large chest.
Men describing what they think women want physically usually reads like someone describing a Tom of Finland drawing. I agree that people generally prefer a physically fit partner.
You start out as strangers but I can say, after attending my local parkrun for a few times I got to know the other runners. Often it's half an hour of chatting at a nice tempo. Though I didn't meet any of them anywhere else, which isn't my intention anyways and I barely ever meet up with people.
You do the activity - you run and chat with some folks if you can. Otherwise, you just go to the social after. A lot of them meetup at a bar or something as the final destination to socialize post-run. Same with socializing before everyone is together and starts running.