Ugh. Proponents of status quo always invoke tradition and use some attractively pastoral but highly marginal examples to paint a white-washed picture that's quite far from reality.
In reality, it's not about the clothes. Muslim men base forcing women to cover up on "hard" facts of exactly the "sun orbits the earth" variety. If you did not think western, you would know that these men think of women as being both naturally disabled (mentally and physically) and inferior in the eyes of Allah (intellectually and morally). This goes far beyond mere decorum and is not at all comparable to office wear vs. bikinis.
Case in point, a few years back I spent a month at an all-women resort in one of the more liberal muslim countries. Once, I happened to swim in front of a local man. To say that he was stunned would be an understatement. His eyes bugged out, he lost his breath, and when he finally managed to speak, he was stuttering. Turns out he quite literally thought that women are physically unable to swim. It was as though he witnessed pigs fly.
What I am saying is, it's easy to take the cultural relativism position until you consider where it leads. As just one example, this is what happens in Saudi Arabia today during rape hearings. A man's testimony is considered fact unless proven otherwise, similar to Western law. A woman's testimony however is considered a presumption, based on these "facts" as loosely quoted from actual Saudi law: women are emotional and as such incapable of sound judgement, women do not participate in public life and as such incapable of understanding what they merely observe, women are forgetful and as such their words are unreliable, and finally, men are by God's will superior and therefore dominate women by default.
So what do you think happens to rape victims when you frame it in those "cultural" terms? By the way, if you think you know where I am going with this, you are wrong - rape victims get sent to jail for the crime of being in the company of a male non-relative in the first place. When you get down to actual lives of actual people, it's not quite as conveniently multi-culti anymore, is it?
So let's not be careful of what we are criticizing. When a tradition is practiced by a whole people, it's cultural norm. When it's forced on a minority that's unable to defend itself - even if said minority has been slowly beaten or acculturated into docile agreement - it's perfectly ok to say it's asinine and just plain wrong.