But if you are a kid that grew up with one parent, we need to have alternatives to help those kids thrive. Harping on the fact that 2 parents are better than 1 does nothing for kids with only one parent (except maybe make them feel defective and disadvantaged).
In contrast, single parent households drastically reduce the chances of success in life along a ton of metrics. Those kids get worse grades, drop out early, suffer learning and mental disorders at a much higher rate, are much more subject to domestic violence, etc. A shocking statistic is that around 85% of people in prison had no significant father figure in their life. This isn’t a one-off statistic either as incarceration rates do seem to track with single mothers over time. It even affects the mothers themselves who report higher rates of mental health issues (this tracks over time and across racial and ethnic groups).
Note: This probably would occur with single fathers too, but fathers are unlikely to be granted custody unless the mother has extreme issues going on and the father has a pristine record in comparison. Likewise, with 90+% of elementary teachers now being women (this wasn’t always true), finding a male role model is more difficult during the early, formative years.
That is what we see, in other countries with more robust (and more redistributive) welfare state systems than the US.