I’ve heard this comment about Batman (and Superman and others) from all kinds of people, from standup comics to Dopers – that he’s wearing underwear over his circus tights.
But … he’s not, is he? It’s just that his circus tights are bi-coloured.
Isn’t it?
I think they’re supposed to be like the trunks worn by wrestlers and circus performers.
In the days before Spandex was invented, this was necessary to prevent the tights from sagging and making the hero look more comical than heroic.