I'm about halfway to 40. No kids. Do not want. I have a little different perspective -- not only do I just not want to "swim upriver and spawn" (sorry, fish biologist) but as a conservationist/biologist, I see and know the effects of sprawl, overpopulation and resource use so I could not, in my mind, ethically add to that. Should I have some sudden flood of oxytocin (highly doubtful), there are literally tens of thousands of children who die every day from lack of care and I would pursue helping one in need.
And I agree with JER -- you can say that there is NOTHING that comes close to your kid-love FOR YOU. But that is individual. What builds love for you over time is in no way an absolute measure of what does the same for someone else. Despite protests, our culture is STILL one of which reproduction is a default. I try not to let it bother me too much, but I wish it would become less of an assumption and more of a well-thought-out choice (which for some it is).