-Having to half-raise my sister, and seeing her be a selfish, spoiled, monstrous brat no matter how my parents tried to raise her--and utterly loathing any children I came in contact with. To this day, I refuse to hold babies when people hold them out to me. No thanks.
-Dealing with all the shitty comments of "you'll make a good mother someday"(when I cook) or "you'll make a good wife someday"(when I sew)--and having to remind them that men can do this too and I'll be oh-so-happy to teach any of my guy friends how to do it, while reiterating that I don't particularly want children but wouldn't mind marriage eventually.
-Knowing that the odds of my child being a bratty, obsessed with popularity whiner is something I couldn't deal with. I was always an outcast, and dealing with a kid who demands expensive clothes and a cellphone at age 7 is something I couldn't put up with. I -might- be able to deal with a child like myself, but again--someone like me would be treated like crap at school, be an outcast, and likely resent me for bringing them into this world just like I resent my parents for doing so. It'd be different if they were into books and stuff, but the odds of them being like that...are extremely low. I have a 99% chance of raising someone exactly like my sister, who gossips, whines, and habitually lies to get their way.
I actually don't know how old I was when I figured all this out--I know it was young, even at say, age 12 or so I wasn't into the idea of being a mommy someday. I had Barbies, but they would always go on adventures I copied from the books I read--they weren't moms, they were explorers and scientists.
I had someone once say that I have 'mom tendencies', but really those are just branded that way because they're "caring tendencies"--I show that I care about people. I ask them how their day was and I'll offer them a caring ear if they're feeling down, I tend to cook and knit and sew--which those are just things I like to do--I also like fixing computers and video games. I've taken care of drunk people before--again, not because I'm a 'future mom' but because I'm a good friend. I'm tired of caring and kindness being attributed to only females. It's highly misogynistic as well as misandrist.
I was about seven years old, when I saw reflected in a mirror how my mother looked at me behind my back. I decided then and there that I didn't want children. I never believed the crap about happiness with babies. I've never regretted my decision. I always had cats but I've never seen them as other than cats.
I'm 75, male, and never had a child. I had 5 siblings and enjoyed them, so I wanted to have quite a few children as well. Another reason I planned on having a bunch was because I was raised in the Mormon faith, which kept preaching that to be happy, you must get married have many children.
My parents were faithful Mormons, but never harassed me to get married or have children. They never said they wanted grandchildren. I think part of the reason they didn't was because 4 of my siblings had children, and 2 of them had many.
I always looked on marriage as a large commitment, so put it off until age 50. Many years before that, I had decided that having children was not for me. It looked like more work than it was worth. As I aged, I started noticing that Mormonism was not living-up to it's promises of happiness, so I didn't feel much guilt for not wanting children.
The marriage ended in disaster, and was part of the reason I became atheist.
I'm very glad I never had children.