Would you let your son wear a skirt if he wants to? Why/why not?

I'd ask him first what's the purpose of wearing skirt. I'll tell him that skirts are only for girls. And that he should dress according to his gender. Maybe, if it's needed for role playing or costume parties, I would allow him. But if not, I'll ask him what's the real reason in calm way and we'll talk about it in a good way.