In my observation of Attire over the years, it occurred to me that it might just be that militarism, and riding horseback to war, were the first, and most significant players in creating this division. Before we wore full body armor in battle, which naturally covered each leg separately, cavalry soldiers simply wore shorter skirts than their foot soldier comrades, in order to allow greater freedom of movement ahorse.
It took a long time for this to take over. It took centuries. Over 1400 years would pass before the dominant form of male dress divided the legs away from each other. By the early Renaissance it was only elderly or scholarly men who wore robes. They were emasculated to a degree by their adherence to activities of the mind, or by their years. Within another 200 years the wearing of robes was relegated to the status of ritual. Robes were worn by judges, priests, and others performing high level, arcane functions. However, under those robes the judges and priests still had their breeches, or, eventually, trousers.
Of course the other major driver in this increased acceptance of skirts on guys is our slowly broadening understanding of the range of human sexuality. We are, inch by inch, coming to see that our ID as sexual beings is not bound by our apparel.