This mindset is extremely disturbing, that the only way to make a difference in the world is to reproduce. Really if you want to make the biggest impact on the most people, you won't bring any more consumers into the world.
Don't get me wrong people who don't want kids shouldn't have kids because they'll resent their kids and likely be poorer parents for it. But claiming they're somehow doing the world a service by "not bringing more consumers into the world" or "reducing their carbon footprint" is just pure rationalization. Kids are not intrinsically a net negative for the environment or society.