It’s biblical; the “population bomb” that Paul Ehrlich warned about is, and was always, nonsense; now, there is a real concern that falling birth rates will have all sorts of dire consequences. Al Mohler comments here:
Another discouraging facet of this whole question is the fact that, as we are seeing in Europe, the Muslims aren’t bashful about having kids. We will live to see, I fear, a heavily-Muslim Europe; the birth rate in Spain, for instance, is 1.3 (this in Catholic Spain? Unreal.); the ‘replacement rate’ (meaning zero growth/zero shrinkage) is 2.1 (have you had your .1 child?).
Seriously, I propose that one of the most important things we can encourage our Christian kids to do is to get married and have lots of kids. And to train them to love the Lord!