This is a relatively recent development, looking at the long term.
As recently as the 1980's nude swimming was standard and doing other exercises nude or nearly so was common in all male venues.
While I did not belong to a gym at that time, I had friends who belonged to either all male gyms or ones that had separate areas for men and women, and they made mention of working out nude and/or seeing others do so. Newspapers noted that when formerly all male athletic clubs went co-ed, members complained that they had to wear clothing when exercising. I have also read about it on this and other boards. Men have noted that at residential YMCA's, men would go to the gym, work out and return to their room without bringing any clothing with them.
The only time I witnessed this directly was on a business trip in the 1980's when I stayed at a hotel that had a gym that local residents could join also. It had separate men's and women' areas. Many of the men were working out nude and essentially all the ones in the lounge area were.
I do daily naked yoga mostly after taking a shower. It is perfect for feeling into the poses.
I have also a stationary bike and when the weather is bad I do naked stationary cycling at home. It is a great motivation to do it naked.
I have never been at a naked gym. But will give it a try at my next resort visit.
I have enough equipment at home now that I don't need to go to a gym. I've bought some new, like the treadmill, but other weights and lifting machines, I've bought used at online auctions or garage sales, or got free from relatives that no longer use them. I would imagine that the majority of exercise equipment that people have bought is rarely or never used. If you have the space, there is a lot of good, barely used, cheap equipment out there.
That said, I am able to work out nude all the time at home. Like others have said, no clothes binding, no sweaty clothes, much cooler, and much more enjoyable. I run on the treadmill nude, but unfortunately, haven't had a chance to run outside nude yet. I live in the middle of 26 acres, with no neighbors close, so I can work out nude on the front porch. But, I can be nude all the time on my property, around my house, and in my big garage.
We did go on a Bare Necessities Cruise in 2019, and they allowed you to use the gym in the nude. It was fun, interesting, and a little weird. They required you to wear shoes, but nothing else. My first time, I was there barefoot, since I'm always barefoot at home, and my feet are pretty tough. An employee came over and told me I wasn't allowed to be barefoot in the gym. Funny, since I was completely nude otherwise. OK, next time I wore shoes. The other weird part was that the crew works out there also and they are not allowed to be nude at all, so they were always clothed. I assume the crew thought that us nude guys were the weird ones. I figure no one knows me, so whatever.
Still not back to the gym mostly because of Covid concerns but also not looking forward to exercising with clothes on. Haven't worn anything for indoor exercise since our grandson went home 5 weeks ago. Now that the weather is getting cooler I have worn a shirt in addition to shorts a few days to walk or bike.
Oh yes, I have done naked yoga ( in a group), naked yoga online, naked fitness dance recently, stretching and in home workouts naked. I have mountain biked naked and even winter fat biked while naked, a little chilly but fun (maybe I'm part eskimo, ha ha).