The answer is very obvious. Government intervention.
Who says the poor can’t afford more children than the rich? Have you take into account political costs and political subsidy?
The poor can afford infinite number of children due to welfare. A billionaire can hardly afford more than 5 due to child support rules that set man’s obligation to be proportional to man’s wealth/income.
In fact, imagine if government says that all chultu worshippers will get subsidy if they have children and all flying spagethy monster will have to pay $50k child support per month per child. I bet chultu worshippers will produce far more children than worshippers of flying spagethy monster.