Sociologist Peter Berger (an ELCA Lutheran), after surveying the overwhelming data that religious people have more children than the non-religious do, offers this explanation:
Religion has always given its adherents a sense of living in a meaningful universe. This protects individuals from what sociologists call anomie—a condition of disorder and meaninglessness. Religion, by the same token, gives a strong sense of identity and confidence in the future. More than anything else that human beings may do, the willingness of becoming a parent requires a good measure of confidence in the future. Mind you, this is not an argument for the truth of religion. Illusions may also bestow meaning and confidence. But my hypothesis offers an explanation for the ubiquity and persistence of religion.
via Why do Godders have so many kids? | Religion and Other Curiosities.