Well, I know what you mean, and agree with you, but there are few exceptions....(refer to below)
Acceptible:
1. Most of these guys are on a pro level, and or look insane. They are exceptionally ripped, huge, or both. I would think this inspiring to some. Granted we are talking pro athletes, or the like.
2.You train at a gym where bodybuilding is taken very seriously, and training without a shirt is accepted. (almost never)
3. You train at a small hardcore gym, with mostly other dudes that are serious, and it's like the Old "Golds Gym" Mecca Arnold Schwartzenegger days. Everybody is just super serious, and the weather is hot as hell most of the time.
4. The gym you train at is exclusively "male" occupied, and geared to. (self explanitory)
Not Acceptible:
1. The guy is fat, skinny, or completely average body type. Just another guy looking for attention, who walks around trying to act bad-ass under 180lbs. Anyone basically, not on a pro, amature, or competitive, or super athletic peak level.
2. Sanitary reasons. Some guys sweat too much and leave machines and Eqiptment disgusting. Unless they clean it up immediately with alcohol, not acceptible.
It would be cool, and accepted i would assume if it were an outdoor gym somewhere, and it was hot weather. that would be cool, but you would have to be in some serious shape to do so, or just everybody was doing it since the weather was so damn hot. It may be considered "gay" also. You may leave the gym one day with a few curious telephone numbers in your locker, you have no idea who they could be, or what for.....(laughs)