I'm married, and have no desire for kids. Nor does my wife. We're happy with the committed companionship and the structure of household life, but are completely happy without kids.
It's weird, though, because all our married friends say we'll change our minds about kids. While we're certainly still inside child-bearing age, we're reaching the point where it would be impractical; maybe even dangerous for her or the kid.
And I have lots of family with either one child or, in more cases, no kids regardless of marital status.
This is not to sound prejudicial, but merely to add anecdotal information to the discussion.
I have experienced that the higher earning, more educated and/or more self-employed my peers are, the less likely they are to have and want children.
My wife and I aren't high earners, but she's a journalist and I'm a self-employed freelance photojournalist and documentarian (we never work together). We're also both very "degreed." In another case, a good friend of mine is a doctor. Married 12 years and he and his wife have no kids. Our accountant has been married for over 30 with no kids (no desire to have them), and my engineer neighbor and his wife have one, with no plans for more.
My parents were both highly degreed and well employed and they only had two kids. From a biological standpoint, they replaced themselves and nothing more. Knowing my brother's interests, it's unlikely he'll have kids either. Could be the death of the bloodline.
Conversely, in public and in the field on the job, I notice more people who seem to be of average education and average or below financial means with several children. Children to an extent that high-earners would be unlikely to comfortably afford. I don't know if the kids are their cause for being scrapers-by, or if there is another cause. And I'm not specifically talking about "welfare queens" having kids just for the check.
I have begun to wonder if there isn't some social factor that keeps the more educated and higher-earning folks from wanting children? Certainly, there are exceptions (this isn't even a rule)
And it's not that we're in a huge, sprawling urban area. We're in a city of half a mil, with a total metro population of about 1M.
If my hypothesis holds water, is it a choice thing, or is it an evolutionary thing? The more you're "worth" from a financial and raw material standpoint, the more you consume from a raw material standpoint. Could this be some self-limiting mechanism to keep our human overhead down?
In my work, I have seen people literally living off a few hundred calories per day and a couple bucks per month and expanding their family. It's not a life I'd want to live, but from an evolutionary standpoint, we don't need computers, rocketships and cubicles. We need to pass on our best genes to the next brood. That apparently requires very little resource to do.