It would behoove them to sit down with a spreadsheet and do some population growth models. It gives you a whole new appreciation for early civilizations development. Also a profound appreciation for what a too-high growth rate does to your technology.
The issue isn’t how many kids you can pop out per year. The issue is how many can you train per year, given a specific matriculation rate.
Too many kids means that the parents are doing nothing else but raising them. Without outside help this means that they are not developing new tech, even if everyone is an Einstein. Neither are they building the new facilities that the burgeoning population needs. Oh, and spend 18 years popping out one new kid per year and this means that the last one is not an adult until 36 years later.
My main char has two wives. They start a new clan. After 25 years, at one kid each per year, they have a total of 47 kids, 68% of which are females who also start breeding at 25. That’s 47 kids being cared for by only three adults. Without outside help, this does not happen.
After 25 years, the daughters start to breed, with a new one coming online per year, and the two original moms drop out (due to exhaustion, no doubt. The moms now have the brains of a turnip.).
At the 50 year mark (500 year life expectancy) you have 323 kids and only 52 adults. This is not counting any mates the males may bring in (polygamous society with no shortage of fresh mates, two to five adults in a family unit).
Work out a few of these models and you quickly begin to apply it to the real world. Africa is poor, starving, and low-tech because of their birth-rate! It’s that simple. The same is true for most other “third-world” countries that are in bad shape. A sign of a too-high growth rate is when the median population age is at or below the matriculation age. This basically means that the kids outnumber the adults.This means that women are barefoot, pregnant, and in the hut (you can’t afford a kitchen with that many kids).
Given this data, the Church’s stance on contraceptives and birth-control is unconscionable.