This is from the FAQ on the site!

So I can wear a skirt instead? or “Hey, no problem, I was gonnawear a dress anyways!”
You’re very clever, but you’re missing the point. Articles of clothing like skirts, shorts, kilts, and dresses don’t count, because people are encouraged to revel in the absence of pants, and not replace pants with other clothing. The point is to relax and enjoy the humor inherent in people not wearing pants.A good rule of thumb is to pretend like you were going to wear pants, and then just fail to put them on.
I think they just don't get it!