Between 1900 and 1930, organized religion saw itself losing control of American attitudes and so began pressing for new laws to support it's point of view. Most or our moral legislation and "blue" laws stem from this period, including the requirement for marriage licenses. There is also the fact that, until 1950 or thereabouts, ALL countries wanted to increase their core population to enlarge the work force and the military pool. Much of the current legislative bias is a holdover from this, I think. Also, consider the fact that a large percentage of our immagrants since 1950 have come from cultures which favor large families. They vote, too. In general, pre-industrial culture favors large families and tries to enforce that bias. Modern medicine plays a part, too. The 19th century was the first time in history that metropolitan birth rates exceeded death rates. Most of human history promotes maximum reproduction. Governments have traditionally tried to keep families intact to minimize the need for social services. Hence, the traditional western family group. It's time for a change, but it won't be easy.