This is entirely anecdotal, however I once was entirely against the idea of having children. I had many justifications; personal, selfish, environmental, social, etc. Though, in hindsight, I probably just didn't want kids.

Right now all I want to do is go home and lay on the floor with my babbling, drooling, high maintenance alarm clock/poop machine. I can't say that meeting my wife made me instantly want kids because we knew each other for a few years before dating, but at some point in time I went from not wanting kids to wanting kids. The conscious choice to have children happened slightly more than 18 months ago, our daughter in now 9 months old. And I should emphasis it was a conscious choice.

I would strongly discourage having children unless you really want them, the negatives will be magnified and the positives will be reduced. For example, going to work after a week of only sleeping 2 hours a night is a lot easier if you can look forward to a happy, two-toothed smile when you get home. If the presence of said smile holds no intrinsic value, then you are in for a long day at work. Likewise, the shear enjoyment of seeing your baby crawl for the first time is soiled if it is accompanied by, "Oh great now we have to baby-proof the lower 3' of the house".

I will grant that I have an incredibly small about of data from a very narrow range of the existence that is parenthood.