I agree with WhatNow: a bluntly polite, "why would you ask such a personal question?" is a legitimate response. And screw what they think about it. A question like that in my book is akin to someone just coming right out and saying, "put me in my place, wouldja?" Ggrrr!
Gardener
"My soul, be satisfied with flowers, With fruit, with weeds even; but gather them In the one garden you may call your own." Cyrano deBergerac