Now you're getting into a Natural Law debate, which is beyond the scope of this thread.
Suffice it to say that according to Natural Law theory, we have a moral obligation as Christians, in the eyes of God, to contribute to the propagation of the species. Furthermore, the primary ends of marriage, as instituted by God, are the bearing and education of children.
If Christian couples refuse to have children, they better have a damn good reason for doing so when they come before the Judgment Seat of God.
Since a fertility rate of 2.1 is required just to maintain a stable population and keep it from crashing, and no economic growth has ever occurred in the context of population contraction, a very good secular argument could be made that, for those able, three children per couple would be a bare minimum.
Makes sense to me.