Ever hear of Halloween? Ever hear of Shakespearean theater (back when women were not allowed to be actors and men had to play the female roles)? Ever hear of Scottish kilts, religious robes, or almost any Middle Eastern traditional garb? It shouldn't be THAT difficult to explain to your child that not everyone dresses the same way, that in many places men wear super-long shirts, and/or that sometimes people wear costumes for fun. Why on earth is that a difficult concept for you as the parent to grasp?
Heck, why is it that you feel the need to drag in adult concepts when a small child asks a simple question? Whatever happened to "I don't know, son, it sure seems weird to me"?!?