The confusion around this sort of moral claim is understandable. Our moral psychology has not yet evolved to solve the problems of today. Humanity grew up in relatively small groups; Rules like “don’t harm others,” or “don’t steal and cheat” are easy to make sense of in a world of largely individual interactions.
That is not our world any longer, though, and our moral sense is evolving to reflect that difference. Moral decisions are no longer about math; Being a part of the solution matters.
The importance of this argument for family size is obvious. If having one fewer child reduces one’s contribution to the harms of climate change, the choice of family size becomes a morally relevant one.