Dancers come in all shapes and sizes, true, but how many have you seen really need a support system? They're almost all b-cup or smaller, and those bust sizes can slip into dancewear and have all the support they need. Hell, a spaghetti-strap camisole is usually enough.

I haven't seen any heroine above teenage with a less than substantial cupsize. Wonder Woman is roughly twice as thick through the bust as she is at the waist (though, at least in her case, her outfit probably is inflexible armor, not pliable latex, and it's a strapless push-up supporter). Point is, those woman are either mono-boobed sacks or they've got undergarments on underneath. That's just the way it works. Seeing as they're not amorphously chested, I argue they are wearing bras et al. and we're just not seeing the wires.

Spandex is pretty goofy, and it leads to problems whereby it makes muscled people look like chubsters. (Dancers are the exception because they are so lean with their muscle. But thicker muscle still pudges-out under spandex.) Some thicker materials, like neoprene (wetsuit-like) would work better while still giving the same general effect without jumping to the thick-as X-Men movie leather.