As pointed out, many conservative Christians believe the mandate in Genesis to "be fruitful and increase in number" is a directive to produce as many children as possible. Whenever I have heard this argument put forward, there is usually a science-sounding adjunct like, "and you know, the whole population of the earth could fit into the state of Texas, each with a ranch house and a back yard." The logic seems to be that as long as there is space to fit people we should keep populating the earth. (This logic was recently put forward by the father on the popular TV show 19 Kids and Counting. I have also heard this from the influential — and controversial -- pastor Mark Driscoll of Mars Hill Church in Seattle).
It seems this odd argument of fitting the word's population into Texas or wherever adds a science-like justification to their faith-based values. While its frustrating that this erroneous thinking is invoked I think it indicates some level of appreciation for science, facts, math, even modeling. A potential response could invoke the ecological footprint concept and point out how much land it would take to feed a population of 7 billion living in suburban ranch houses.
