The general rule seems to be that if they feel they need to ask WHY you're wearing a kilt, then you can't explain it to them.

I've had a bit of success with diverting the question. Raised eyebrows and a smile, and "Why do you ask? Do you like it?" puts things onto a different track.

Or if they're really obnoxious, say "I wore it especially to get you to notice me, and come and talk to me." Growl it, and then lick your lips.