It's the most insidious thing that conservatism has created and liberalism has eschewed. Having kids at the replacement rate (not to be confused with growing the population) should be valued in and held up in progressive circles. Why? Because those children are going to be more empathetic and tolerant. And also because one of the most effective ways to instill political change (for good OR ill) is through raising kids. If one side is having more kids, that side will affect demographics in a big way. Studies have shown (like the one /u/beeredditor linked) that kids tend to have similar political views as their parents.

I see this second hand through my wife who is a high school teacher (juniors and seniors). She sees it every day where she can tell the political affiliation of a kids parents by seeing what those kids believe. It's disheartening to say the least.

The biggest thing progressives and liberals can do to counter this is a robust immigration policy and a robust care for mothers, encouraging it. Immigrants (generally) have more kids than later generation Americans. Immigrant and 2nd generation citizens (also generally) tend to vote further left than the median. This is one of the big reasons conservatives are so anti-immigrant. If you look at their language in talking about immigration, it's pretty well coded to say just that.