Yglesias is wrong here:
That the two coincide frequently is no coincidence, but there are plenty of programs that liberals once favored, such as the old welfare system, that seemed socially beneficial (paying single mothers so that they could raise their children) that ended up being massively destructive to families. I can easily imagine some forms of social insurance and redistribution do not promote marriage or negatively affect it.
Why does this matter? Because married parents are far, far better for children, especially ones not born to the upper classes. It is a good for child raising that is distinctly non monetary.