I feel like it's hard to say what's 'natural' without excluding the effects of social pressure and norms, which are tremendously powerful.

It's like when you hear politicians say that everyone aspires to own a home - well, sure, I can see how it might look that way, but that's only because we've set up our economy a certain way and it's constantly reinforced through the media, it's not hard to think of a system where that isn't the case.

I'm guessing it's not a coincidence that many of the people I know who've chosen to be child-free are also non-conformist in other ways, and generally don't give a stuff what society thinks.