At 13, I worked at a day care center. That completely killed any desire of wanting kids. I still say it's the best form of birth control ever.
At 21, I met my ex-husband. He always wanted children. I warmed to the idea of having kids, and did lots of research. But we were never in a situation where we could afford to have any, and the marriage also made it clear that I would end up being the responsible one who'd have to take care of two children - our baby and my ex.
At 30, (while still married) I firmly decided that I did NOT want children. He said that I would 'change my mind later'.
At 33, we finally divorced without any, save for a Malti-poo and a huge calico - he got custody of the cat while I kept the dog. (Although it was cute how we referred to catfood and dogfood as 'baby food'. Used to freak out the grocery store clerks.)
So, I'm definitely not in the market for having children, and that will be part of the package for anyone else I'm with - they must understand that I will not have any. Of course it seems most guys do want children (or at least proof of their virility), so my options are very limited.