As for the men who feel they have to publicly tell everyone what effect seeing women in certain outfits has on them, please keep your lust (or lack thereof) to yourself. What a way to talk about your sisters in Christ! Yes, Christian women have a duty to dress modestly, although there seems to be a great deal of confusion over what exactly that entails. As I was discussing with a friend yesterday, ideas about modesty have changed over the years. It used to be considered the height of immodesty not to wear a corset. One of the reasons the English imputed immorality to the Irish was because the Irish women were corset-less. Marie-Antoinette was considered a loose woman when she went without a corset. It also used to be immodest to show one's ankles but perfectly acceptable for ladies of the world, and a matter of etiquette on formal occasions, to wear a low cut dress. Ideas of modesty have indeed changed.
As anyone can see, slacks, as long as they are not skin-tight, are certainly more modest than mini-skirts or certain revealing dresses. I understand how some people feel uncomfortable in skirts and are unable to work in them. Since I was a nun for five years in upstate New York I know how to do all kinds of work in long skirts. I have shoveled snow, scrubbed floors, climbed ladders, and gardened. However, other women who are not used to skirts or dresses might find it awkward to work in them or even go places. They might trip; it could be dangerous. The clothes have to fit the duties at hand. That's how I see it.
Cardinal Siri, however, made some excellent points in 1960 when he composed his Notification Concerning Men's Dress Worn by Women. It is not dogma and was never even meant for public consumption but worthy of consideration nevertheless. The Cardinal did not think slacks on women were immodest but feared they were a symptom of the eventual and overall masculinization of women to the detriment of their role in the family and in society. In many ways, his words were prophetic.
More HERE and HERE.