See, that's the problem. You're sexualizing a woman's ass. A woman doesn't automatically consider her ass to be something sexual. Straight men do. An ass is an ass, and the fact that a man finds my ass sexual is not my concern. Nor do I want to be told about it. But I do like how that skirt looks on my ass. For me. My comfort is the focus, not somebody else's hard-on.
I clearly don't deny there's sexism ingrained in most aspects of society, but fashion was not the greatest one to cite on your part. Most high fashion is where risks are taken to challenge what "sexy" (heterosexual man's version of sexy) is, and has less to do with being "attractive" and more about being "artistically beautiful." Does this turn you on? http://2.bp.blogspot.com/_YPLSyaja5vU/R7Xqwvpw1YI/AAAAAAAAAh...
And lastly, to say that caring about your appearance for self-confidence's sake is really just latent sexism, and that women are unthinking enough to not know they do everything for men anyway is... well, first silly, second, offensive, and third, incorrect. We really need to get away from men telling women that their behavior is "ingrained."
Their concept of "looking good" is shaped by magazines and tv shows (still much more so that men, on average); and if you look at your average magazine, you'll see oozes of sex emanating from advertising and content. The concept of "looking good" in western society is equivalent to "looking sexy/desiderable to the opposite sex". Why would women's skirts emphasize one's ass otherwise? When in a suit/tailleur (pardon me, I'm not a tailor), it's among the most professional attires a woman can wear, and it's been sexualized to high heaven. Why? Because it makes women feel powerful. And why do they feel powerful? Because they can attract men, and that's what society tells them their power should be.
So whoever tells herself that she wants to look good "to feel better" is, unconsciously, accepting sexism in society almost as much as one who'd crack a joke about it; she's just doing at a much deeper level, one which women still refuse to deal with (it would mean dropping the whole "fashion" industry like a hot potato).