Isn't it amazing that people who are not intimately acquainted with you think they have the right to comment on your choice of how many children to have? But they do it. A lot. This is not just a Childfree issue. People who have any number of children take a lot of flak, too. (I have a friend who invariably comments "They know what causes that now" when she sees someone with several young'uns in tow.) But this is a site for childfree people, so this is a list of snappy comebacks when confronted by people who refuse to believe you do not want to have any children. (Be warned: some of them are pretty venomous!) These were gleened from alt.support.childfree, and credit is given where it is known, but some of these answers have been bandied around so much, it's hard to tell who was first to suggest it.

And without further adieu...

I did have children, but my dog ate them.
I don't believe in species mixing.
I'm afraid they might grow up to be like you.
My religion forbids it.
The sex change left me only partially functional, never mind reproductional.
I'm allergic to snot.
I hate the little buggers.
Dana Kelman

It's a condition of my husband's parole that he not reproduce.
Willie & Bean Kruer

I love children! Especially with ketchup!
"Frenchy"
Corollary: I had some. They were delicious.

My cats are allergic.
Susan Mitchell

They're my genes, and my mommy says I don't have to share.
Kandi :)

I have all the pets I need.
Mark Ellsworth

The popularity of this answer is largely due to its double entendre.

Jan Harris, who learned it on a.s.c., used it with the following results:

"You mean you can't have children or that you don't like children?" she asked.
"Take your pick!" I said, brightly. It was the first time I had seen anyone look completely nonplussed.

When are you going to have some manners?

As soon as I have my brain replaced with a cauliflower.
When I have a labotomy.
Mark Ellsworth

I gave (him/her) one once, but (he/she) just threw it away.

Some people are so confident that you don't know your own mind. Granted, people do change their minds about lots of things. Some people who thought they did not want children have changed their minds, just as some parents regret the decision to have children. But this does not justify a casual acquaintance's insistence that you'll change your mind.

Fortunately, there are answers for that, too.

If I have a child, and discover that I was right about not wanting one, are you going raise him/her?

CBC: Did you want to have children?
BB: Yes
CBC: You'll change your mind

It's too late for that.
(How do they know if you've had an operation yet?)

The Washingtonian did an article on childfree people a while back. In it, they asked people who have children why they had children. The number one answer?

So, try to have some patience with folks who don't understand why you are childfree. They may not know why they are not.