Interesting thread, this. I'm both disappointed and reassured by some of the comments here.
I don't want kids. I've never wanted kids, and at 42 and counting I'm confident that this isn't going to change. People bring their kids into work sometimes; the (largely female) half of the office grinds to a cooing halt, my reaction is "take that bloody thing away from me" and I'll go and find somewhere else to be. I simply don't have the parental gene, there is no child-shaped hole in my life and the ever-so-predictable bleating of "but WWHHHHYYYYYY?!" isn't going to change that.
It's a similar reaction to the one I get sometimes when people find out I'm vegetarian. I've been veggie for over half my life, and in hindsight I never really wanted to eat meat even before I knew what a vegetarian was. So to suggest that I'm somehow in a massive ball of denial because I saw a dead cow when I was six or some other hokey logic, and that I might wake up tomorrow with a sudden craving for a Big Mac with extra bacon, is frankly condescending.
A couple of points:
1) Just because I'm veggie, or childless, does not mean that I expect you to be. You don't need to get defensive and you don't need to justify doing something different from me. You don't need to tell me how you couldn't possibly be vegetarian as you love chicken too much, triumphantly point at my leather shoes and tell me I'm doing it wrong, or try to persuade me that removing crayon from the carpet and baby sick from car upholstery is your favouritest thing in the whole world. I didn't ask you to do any of those things; you want 2.4 children you can take to a steakhouse, knock yourself out, I don't care.
2) Similarly, I do not have to justify my lifestyle choices to you. The answer to "why" is "because I want to." Do you honestly think that in the four decades I've been on this planet I've given less thought about my own life decisions than the 30 seconds you've just given it? Bog off and leave me alone.
I do take the point of course that any irreversible decision needs to be thought about very carefully. Having a foot high back tattoo of Walter White from Breaking Bad might seem like a fantastic idea when you're 18, but it's going to look like Michael Jackson's sat too close to the fire by the time you're 80. So comments along the lines of "I never wanted kids, but I changed my mind" are helpful. But you need to understand and respect that for some people "not wanting children" actually really is an absolute stance.