Seems like when I first got married there were only one or two NFP methods available. Now there are four or five competing methods and the technology just keeps improving.
The state of the "technology" now is VERY accurate and effective when used properly (unlike the frequently joked-about old "rhythm method"). NFP methods are based on recognizing the body's signals of fertility and then either abstaining or not during fertile times, depending on whether you want to achieve or avoid pregnancy.
So NFP could be taught all over the world, even in impoverished third world areas and if people wanted to reduce the number of children they have they could do it.
Boy, that problem was seemingly easy and cheap to solve.