Being in the Southern US wearing thongs to the Gym at most places is frowned upon. We have those good southern baptists who thnk you should be covered. Hell we have trouble at somplaces with spandex, I’m jealous of all you guys who can wear them
Most gyms anywhere will say no. I was lucky enough to find one that didn’t care. Go to an aquatic center. Places focused on swimming usually have no issue
I swam 2 km today at my local outdoor council pool. I wore a new thong I had just bought. It is Vaquita, and it was their Ryker buckle thong in Calypso material. It does not have a draw string, so I was not sure how well it would stay in place. Thankfully size medium was tight enough to hold in place, although the back did roll over a bit during tumble turns, and when I exited the pool. The colour and style are great. I got a nice smile from a lady who exited near me. I had a lane to myself for half my swim, and one other man for the second half. I recognised that swimmer as the Mayor of our city.