This statement would only make sense in a weird Huxlean utopia wherein all men and women dressed alike. Name me any male who dresses like that, whereas most young, attractive women dress like that regularly (at least if you live in a semi-warm climate).

Uh?

Most men that I know will wear shorts when it's hot out. Sometimes they'll even wear tank tops or go shirtless!

Oh wait...

In the summer, men wear short pants because it's hot, not because they look good. Men in short pants look like overgrown, hairy toddlers.

I think that statement is hilarious and I think that you have issues.

But let's go back to men and women dressing alike or not. In many athletic, physically demanding professions, men and women DO dress identically. Racecar driving. Scuba diving. Skiing and snowboarding. The military. Police work. Firefighters. Doctors. If being a superhero is the a combined athletic/law enforcement endeavor, then why shouldn't both men and women in general wear the same amount of covering? They do in real life, in those types of professions.

Superhero costumes reflect social mores about clothing in general,

And if the social mores about clothing are sexist, then superhero outfits are going to be sexist. Yes. I know. That's the point.

I don't know why it should be surprising

It's not surprising. That doesn't mean that it's not annoying.