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.
Thanks for the reply, although I wasn't asking for a secular argument; in fact, the secular argument is frightening. Would you support legislation requiring couples to produce three children?