Simple solution is stop abortions & promote adoptions. I'm serious in this. I would say based on married couples I see around me the actual number of couples choosing not to have children is actually very low. We've all seen the numbers on abortion though which is already life conceived. Their genocide is a major problem and an actual sin. But it's not married men and women simply choosing not to have children that is causing a decline nor are they sinning in doing so unless the Lord himself places it upon their hearts to have kids and THEN they refuse.
I don't see how even by a cursy look around that you can't see how few "white"families have more than 2 children. Forty years ago, in any public gathering there would have been many more large families in evidence. It wasn't called the baby boom for nothing. Twenty years before, during the depression, the birth rate was lower, but the postwar prosperity at first led to the creation of many large families. It was the sexual revolution and the separation of sex from child bearing that led to the drop in the birth rate to a level below that of the depression rate.