I spent an afternoon at a coffee shop near the big local college recently. There was a girls' club meeting in the shop, and fully 90% were wearing leggings, chunky boots and something else. At least 3/4 were not fully covering their rears/fronts, and of those I could have easily told you what color and style more than half of their underwear was (not that I was trying to see, but it was mostly neon and skimpy). So 20+ girls who I asssume were 18-21 years old, showing me their underwear on a Sunday afternoon. I just plain do not get it. Some of the leggings were thick, but you could see panty lines anyway, and others were careful to cover with tunics, skirts or even that sort of unusual shorts-over-leggings thing, which I have no problem with in terms of propriety.
So while I fully agree that sexism has no place in a dress code, I also understand urging people to be more conscientious about this particular trend, and I understand putting limits on it if common sense is in short supply.