"Their book suggests that many women don't marry the father of their child not because they reject the concept of marriage, but because they do not see him as a reliable source of economic security or stability."

Marriage is not a magical thing that fixes economic problems, even if you want to keep turning the clock back and make certain acts outside of marriage illegal. The only thing that fixes economic problems is money.

"Kearney says she wants to grab the attention of both conservatives who say they care about children's well-being and progressives reluctant to talk about family structures, because the link between single parenting, inequality and mobility in America is too strong to deny."

Marriage is not a magical thing that creates good family structures, and parents can be deadbeat parents even within a marriage. It might give people motivation to stick around, but families that "stick together for the children" can have A) abuse, and B) the same sort of economic problems as unmarried if one or both parents aren't working. What does enable good family structures is support, provided by extended family or other things.