One day, someone asked the Pope -- Pius XI, who ruled from 1929 to 1939 -- and on January 12, 1930, in a very explicit instruction issued to the bishops of the world, he told them.
We recall that a dress cannot be called decent which is cut deeper than two fingers breadth under the pit of the throat, which does not cover the arms at least to the elbows, and scarcely reaches a bit beyond the knee. Furthermore, dresses of transparent material are improper. Let parents keep their daughters away from public gymnastic games and contests; but, if their daughters are compelled to attend such exhibitions, let them see to it that they are fully and modestly dressed. Let them never permit their daughters to don immodest garb.
The directive of Pius XI was reiterated by his successor, Pius XII, in 1956, speaking through his Cardinal Vicar, when he said: "There always exists an absolute norm to be preserved."
More recently, in a Notification on June 12, 1960, Guiseppe Cardinal Siri expressed concern over the wearing of men's clothing by women, specifically trousers. He was especially disapproving of trousers that were form-fitting on women.
To respond to the above, one would be curious as to what that "absolute norm" would be, especially since no basis in Objective Truth is cited in the above application of the norms, and the ability to incite the senses is often conditioned by the culture in which one is raised. There was a time in Europe when a woman showing her bare ankle was considered scandalous, a factor unmentioned in the papal decrees. (So, at some point, bare ankles became acceptable, right?) What's more, a man who was raised in the South Sea Islands, or in some parts of Africa, may not be given to impure thoughts when seeing bare-breasted women, especially if he has seen them all his life. Further, to bar women from wearing pants assumes that pants, by their essential nature, are inherently male garb. Given the whole of history (and we're dealing with millennia here, and all parts of the world), this is a relatively new concept, especially in the West.
Perhaps modesty is like its nemesis, pornography; we know it when we see it. Modesty may also be conditioned by time or place. A sleeveless evening gown which may be very becoming, and nothing more, may be just distracting enough when worn in Church. And so the woman might veil her hair -- her "crowning glory," as my mother used to say -- because one honors the mystique of that which is veiled, much as the priest veiled the ciborium before returning it to the tabernacle after Communion. We don't worry much about veiling our women in this part of the world, do we, fellas?
It is also helpful to hear from young men and women themselves, those who are learning to come to grips with the discovery, the mystery that is human sexuality. Amanda of Modesty-Mall.com would remind us that these are not theological truths, so much as a guide for us, a reminder that what is being covered is reserved for a higher purpose, and that the mind should consider such outfitting accordingly.
(Video courtesy of Gloria.TV. Photos courtesy of Modesty-Mall.com. Skirt/pullover ensemble by Clare R. All photos used without permission or shame.)
POSTSCRIPT: Personally, I can always count on Wikipedia to do justice to the topic.