The Freakonomics guys recently had a post about how children are basically an addiction for parents - they create surges of oxytocin at unpredictable times by occasionally being nice, so the parent puts up with all the negative behaviour in order to get the rewards when they say "I love you".
Having 2 small (somewhat rambunctious, shall we say) children, this makes a lot of sense to me.
So perhaps the issue is that when the rational brain takes over it is more powerful than the chemical addictions. This could also then apply to non-parents (i.e. nobody would get "broody" when seeing other, well behaved children, becuase the chances are they would have also seen kids behaving like monsters). One would assume that the people on the drug in the story would also not get involved with other (addictive) drugs - not having kids would simply be an extension of that.
Must try and get hold of that story though ... especially as I've always been a Pohl fan!
Edit to include link: http://freakonomics.blogs.nytimes.com/2010/11/16/the-parent-trap-addiction/
What would happen if people could feel what they knew about probability? The story's in Gateways, a tribute collection for Frederik Pohl. Unfortunately, the story is more about whether a drug with that effect is a good idea rather than extended exploration of the effects, but it's still interesting. One of the points is that, while people on the drug are more sensible (laugh at lotteries, don't eat food that makes them feel bad), they don't have children. That last might be less of an obvious outcome than it seems-- rationality can improve parenthood.