Here are some comebacks:
You haven't asked yet.
I was hoping to do something meaningful with my life.
Because I just love hearing this question.
Just lucky, I guess.
It gives my mother something to live for.
My fiance is awaiting his/her parole.
I'm still hoping for a shot at Miss/Mr. America.
Do you know how hard it is to get two tickets to Miss Saigon?
I'm waiting until I get to be your age.
It didn't seem worth a blood test.
I already have enough laundry to do, thank you.
Because I think it would take all the spontaneity out of dating.
My co-op board doesn't allow spouses.
I'd have to forfeit my billion dollar trust fund.
They just opened a great singles bar on my block.
I wouldn't want my parents to drop dead from sheer happiness.
I guess it just goes to prove that you can't trust those voodoo doll rituals.
What? And lose all the money I've invested in running personal ads?
I don't want to have to support another person on my paycheck.
Why aren't you thin?
I'm married to my career, although recently we have been considering a trial separation.
Bonus reply for Single Mothers: Because having a husband and a child would be redundant.