Sorry, but as a stats person, this kind of logic makes my head hurt. Lets examine with with a similar scenario, same logic.
“I did a study at the local university. Turns out, over 99% of them took the SAT. Looks like we just need to make sure that high school kids take the SAT, and they will do great in college!”
NFP does not make a happy marriage. NFP users, by a huge majority, are dedicated, loving, steadfast in their faith, and incredibly family focused. You have the logic backwards. It is important to understand why this distinction is drawn.
It’s not really a “similar scenario, same logic.” In the example you’ve given, the SAT is a requirement for college. Nearly college students have taken the SAT, therefore, nearly all
top college students will have taken the SAT. So the correlation/causation mistake here is in looking to the smaller sub-category among two related categories. However, what you’re trying to demonstrate is the existence of potential lurking factors (e.g., family focused, steadfast in faith, loving, and dedicated) that explain
both the practice of NFP and the low rate of divorce, making them correlate with one another. It’s much more like the “ice cream sales are correlated with crime” example, where the lurking factor is summer.
The problem is that I’m not sure that the lurking factors with NFP and divorce are just
purely lurking. Sure, love, faith, will-power, and focus on family both underlie the low divorce rate AND the practice of NFP (just like summer underlies both crime and ice cream sales). However, unlike “being summer,” love, faith, will-power, and family-focus are not binary, categorical qualities and they can all increase in degree. Many will argue that, because of this, “practicing NFP” and “having love, faith, will-power, and family focus” are factors that play on each other. Thus, a couple that has love, faith, will-power, and family focus will be more likely to practice NFP, but as that couple practices NFP, they will grow in love, will-power, family focus, and faith as it re-affirms those qualities. Since love, faith, will-power, and family focus underlie a low divorce rate, it seems (logically) that practicing NFP could have some causal force. Because of the lurking factors, of course, the statistics aren’t quite as impressive as you’d think, but, as I’ve said, the factors that lurk behind NFP might also increase with NFP, so there is some value to the statistics as well.
Edit: Just wanted to end with another analogy. Arguing that “couples who practice NFP have more self-control (which would be properly correlated with a lower divorce rate)” is similar to arguing that “people who go to church have more faith.” Sure, it takes some amount of faith to go to church initially, and (for the most part) only people with more faith will continue to go to church, but, it also goes in the other direction in church can reaffirm and increase the person’s faith. So, as as a person continues to go to church, they’ll grow in faith, but as they grow in faith they’re more likely to go to church. The factors share a complicated relationship.