Leftwing social critics of the modern tradition describe social norms as arbitrary, especially as they relate to gender. They (henceforth, “leftists”) have an equally noticeable tendency to be wrong, and I bet the matters described herein are no exception.
First, the word “arbitrary” denotes that which happens without reason. With leftists, it means, “For reasons that I should not be expected to care about,” so obviously I will not bother to argue against that. I will give some possible reasons, and then people’s moral proclivities be what they may.
Men and women have different clothing norms almost everywhere, with exceptions, just as there are exceptions to the proposition that nodding of the head up and down is an affirmative gesture. The latter is overwhelmingly common in a vast range of cultures, but it does not follow from its being less than totally universal that it is arbitrary. There is some reason that the tendency exists, even if it is just as simple as copying a high-status group.
Likewise: clothes. Dresses and skirts are strongly associated with femaleness and trousers only weakly with maleness, to such a degree that a skirt on a male is sometimes seen as an aberration and trousers on women not. Some have tried, without lasting success, to change this.
It is questionable how common clothing styles now associated with women have been on men for much of history. The kinds of “skirt-like” or “dress-like” attire that have been somewhat common among men at various points include pulpit robes or cassocks, which have specialised uses; they are not everyday attire for the wearers, and many are relics of the Roman tradition, which I will get to later. Indeed, trousers were apparently common only on men in medieval England, and certainly among the Germanic tribes (Didorus Siculus, V, section 30) and Persians. Charlemagne (Laver. pp. 52-3) donned the tunic, which is dress-like enough, for ceremonial reasons and otherwise wore trousers.
Nevertheless, tunics and kirtles were easy to see on both sexes when they were popular. So one could easily get the impression that trousers and trouser-like garments have become less gendered through time, whereas skirt-like and dress-like garments have become more so. What changed?
The reasons people wore particular types of clothing were different in the past. Practicality, cost, class divisions, and social conservatism were high in the past. Setting aside the thorny question of how knee-length vestments and tights became high-status among European males of the upper class, one can look at the Romans. Braccae (woollen trousers) were associated with barbarians and trousers were seldom worn by Romans. Nevertheless, soldiers did use trousers when practical, especially in the cold regions of the empire. Rome held its clothing conventions in high enough esteem that, after the Empire’s collapse, they were still followed for reasons of tradition qua tradition, and status.
In many professions, especially in the industrial era, skirts and dresses would have proven impractical for a lot of work, most conspicuously among working-class men, and to the extent that this was true, wearing those garments would have been associated with impracticality and therefore low status. The examples are fairly obvious, but this is of questionable relevance to the present day when the majority of work is in the tertiary and quaternary sectors.
It looks almost as though before the 1800s some men wore trousers but far fewer women, and trousers were not “in” as the default male fashion choice until the 1800s.
Finally, perhaps most saliently, clothing for purely ornamental purposes was rare historically outside the elite. Aristocrats especially the French, and Georgian-era gentry, were known for it, but never the average person. This changed in the 20th century, especially the second half. Interestingly, trousers started to become fashionable among women in the West at about the same time that short skirts did. For trousers, I would guess it was a matter of practicality and, secondarily, dissociation from any historic tradition, e.g. that of the Romans or the Catholic Church. In the latter, changing sexual mores (hence, miniskirts) and social mores; the trouser’s association with work, an essential part of the male sex role, came to embody a kind of archetype which some women sought to copy as the 20th century went along.
The association of long (e.g. ankle-length) skirts and dresses with chastity was probably not as strong in the early 20th century as later, because there was less in public life with which to contrast it, i.e. the miniskirt-wearers were barely present.
Today, almost the only reasons to be choosy about what one wears are aesthetic, whether sexual or not. Thus, the vast majority of men do not even wear shorts unless 1) the weather is unbearably hot or 2) they are performing some activity that necessitates it or makes it easier, e.g. running and swimming. No one is interested in seeing men’s legs per se except homosexual men. By contrast, women’s legs are objects of intense desire and adoration for legions. So, in the present, a man who wears a long skirt or dress is giving off signals of chastity or sexual innocence, which is ridiculous in men. If he wears a short skirt or dress, he is giving off signals of sexual attractiveness, which, again, is absurd; the visual advertisement of these qualities is nearly meaningless in men: especially for chastity, but even for attractiveness unless he is profoundly physically attractive.
Other social changes have come about along the same course for comparable reasons, such as the practice of leg-shaving, far more common in women than men, and in men it is typically to highlight musculature: athletes, swimmers, models, etc.
One finds oneself suspicious of anyone claiming that a social trend emerged from the aether simply because of marketing or propaganda. The evidence that propaganda, after controlling for confounding factors, affects public opinion is thin. It is not even true of Hitler’s speeches. There are always confounds: some economic, some endogenous and innate. It is sometimes claimed that the preference for shaven legs came about in the early 20th century in response to specific ad campaigns, which explains why one sees loads of old paintings of women with visibly hairy legs. At least, it would explain that if it were true.
Few women have their legs shaved the year round unless they live in a climate wherein they can expect to have their legs bare on any given day. In the past, when women seldom used their legs as sexual ornaments, it is reasonable to deduce that shaving was even rarer but became common once they did commonly use them for that purpose. Are we really to believe that this is a coincidence?
Women have sparser leg hair than men to begin with, which is a neotenous trait along with lack of facial hair, lower height, paedomorphism in facial structure, etc, all of which are considered highly attractive in women. Since relative lack of hirsutism is a sex-typical trait in females and the heightened neoteny that shaving projects is attractive, women who frequently have their legs bared shave them. This is descriptive, not to say that anyone of either sex is ethically obliged to be attractive. However, what constitutes an attractive feature is fairly universal.
Tangentially, something similar occurred to cause the gradual skin-lightening of Europeans. Women almost universally have lighter skin than men, and more sex-typical features are preferred in mates. Europe is thought to have had a female-skewed sex ratio for much of its prehistory, thus increasing competition among females for mates and upwardly modulating selection upon elements of female sexual attractiveness, many of which spilled over into males either as byproducts or due to bidirectional sexual selection. This is one reason among many why Europeans are the most attractive race.
All this could be obvious. Much of it may have been once. Alas, few have any interest in finding the knowledge themselves.