'Regular shorts that extend an inch or two below the crotch' is close enough to Robin-style for me.
And the main difference I see between men and women going shirtless - and I agree, it's absolutely a double standard, but still - is that with women, it's more because of society's increasing tendency towards skimpier and skimpier female outfits, so one could argue they're being fashionable and carefree. Men, on the other hand, don't have this excuse. All kinds of women (well, young women) wear very little up top, but with men, it seems to me that most of the guys who go shirtless are the types who are obsessive about showing off EVERY SINGLE MUSCLE, and those types tend to be... well, shall we say, not the most humble people in the world. Women tend to focus more on simply looking toned and slim and youthful, which looks great without a shirt, but the guys who go around shirtless are almost always the bodybuilder types. In other words, I have no moral objection or anything to guys going shirtless; I just don't find it a particularly good indicator of character - I mean, if you're THAT muscular, it'll show anyway; taking things a step further is just being a strutting peacock.