* Children, or even a single child, would chain me down in ways I'd hate; it's 20 years of your life down the drain, dedicated to bringing another human being in the world, and when you're done, you're old. It's possible to abandon it, of course, but I'd hate to bring a human being into the world just to ruin its life by throwing it into the system. I adore my freedom and my life, and the things I do and plan on doing make me happy.
Cost vs. benefit. A child would cost approximately 20 years of time and >=$350,000, and the actual benefit of that is unknown -- while a child could grow up to be great, statistically it will be average. I'd rather put those 20 years towards both improving myself and contributing in any other way I can.
They're annoying. Early on, they scream, they poop, they piss, they stink, and they wreck everything you like accidentally, even if you try to child-proof your house. As they get older and become teenagers, they get unruly, and become rebellious little shits who no longer wreck everything you hold dear accidentally... they do it purposefully.
I'm very selfish; dedicating my time to it would be difficult as there is almost anything else I'd prefer doing than raising and teaching a screaming little monkey. And carrying a lifeform in your body for nearly a year? Why would you do that to yourself?
My genetics are horrid, on both sides. What if the child turns out like my mother? My father? My sister? My grandparents? My aunts and uncles? What if I have it and it goes the way of so many others in my family and kills itself? What if it has heart problems? Schizophrenia? Addiction troubles?
I'd hate for it to be a disappointment, and it being a disappointment would make me despise it. I'd have unreasonably high expectations for it, and I've seen what that does to people.
There is not a single 'parent' bone in my body. I've never wanted children and I don't understand why someone would want one. I have no urge to reproduce.