The problem solves itself; the societies that are pro-natal will survive and the ones thay are not will die out.
That assumes that offspring follow their parents' ideology directly. While many may, some will not.
What we've seen over time is a defection from e.g. religious right upbringing, especially as offspring move into denser urban areas.
But two parents who feel strongly about something having children will produce children with a certain biological proclivity.
It’s a lot to bet that people will deny their own biology.
Yes, but aren't there biological proclivities to certain personality traits, and then proclivities of people to certain personality traits to political ideology?
So if liberals don't reproduce, it could start selection against the "openness to experience" personality trait.