Insty points out that much of the financial "inequality" in the US is due to the marriage gap. Wonder of wonders, even the NYT is finally willing to discuss the forbidden topic: Two Classes, Divided by ‘I Do’. It's not news. Sen. Pat Moynihan famously warned about this decades ago, but the subject was judged to be politically incorrect.
Why is it a forbidden topic? I can think of at least two reasons. One is fear of the racial aspect, although no race has a monopoly on single motherhood. My second reason is the knee-jerk refusal to judge based on bourgeois life style choices, or some such Marxist baloney.
Even with government functioning as a modest trust fund for many, having kids outside marriage is not a recipe for a good, orderly life (from my petit bourgeois standpoint, of course). It's not mostly about income - it's about functionality, support, structure, partnership, responsibility, committment, teamwork. This is not a secret. The income part is just an extra side-effect if two people have jobs.
As best I can tell, it takes at least two people to raise kids properly. Ideally, parents, grandparents, aunts and uncles, and close family friends around too. Nannies or babysitters if you can afford them. Tribally, in extended family and in community. Income doesn't really matter much. Kids thrive on Cheerios, Wonder Bread and baloney, macaroni and cheese, hamburgers, and Fluffernutter. I did. 6'2" and fairly strong.