As others have said the main issue is the sweat going all over the place and making the machines and area dirty. If they had some kind of floor or material that absorbed it then it would work. Also ive worked out nude and as a man my little penis bouncing around can hurt when my balls slap into my thighs and its more comfortable to have undeerwear on as a man. I feel that women might have a similar issue with their breasts, again there is a reason clothed exist. But i think that a clothing optional gym could work maybe. Id probably just wear underwear in that case.