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

How old is he? I noticed that toddlers are fond of using things of the opposite sex. My 3 year-old son does that too coz I think he's amazed with girls he sees on videos having long hair and nice dresses. I think it all started with Frozen. But I'm not too worried about it because we're orienting him well on how to behave like a boy. And he doesn't do it all the time so no fuzz, really.

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.