Some of the "best" responces are also a good way to get into a fight. Consider whether or not you REALLY want to go down that road.

With that caveat in mind, I once told a guy that he was confused. A Kilt is what a man wears.
A skirt is what his girlfriend leaves wadded up on the floor when she meets a man in a kilt.

A more civilized response comes from a signature I've seen on this board (and I apoligize in advance that I don't remember who had it...)
It was
"Son, someday you will make a girl very happy for a short period of time.
Then she'll leave you and be with new men who are ten times better than you could ever hope to be.
These men wear kilts."