Bike shorts work pretty well so long as they are never worn by themselves. MY UA and similar boxer briefs work pretty well most of the time.
However, I swear one of the boys was wearing a pair of Lulu shorts the other day and some other yoga type stuff. One of the boys around 7 or 8 asked if he was a girl in the boy's class
He probably didn't chafe but still...
And no, we have a policy of never wearing our gymnastics singlets in gym unless it's for meets. I'm glad this gym is like this so I don't have a bunch of boys in unitards/singlets. However, these guys barely understand the idea of working out shirtless for some reason. Nobody is old-school, anymore.