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 fiancée 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.