The Conservative argument is that the family (specifically the relatively new judeo-christian family, since Old Testament figures routinely had many wives/concubines) is the foundation of society, and any deviation from that would spell disaster in the long run.

(It somehow did not spell disaster to these old testament figures who were, to all accounts, quite Godly and had many wives, but hey. Times a-change.)

It tracks pretty well with the general Conservative view that America's success is due to a specific godliness and being favored by God, rather than specific secular policies that have been implemented. That is the reason a lot (not all, but a lot) of conservatives seem so free market at first; really, it's less being pro-free-market as much as thinking the real important things like gay marriage, abortion, and other social issues will make or unmake America.

As always, those views are subject to alteration or being disregarded by individual conservatives, but if you track conservative/christian theological thinking.

A great review of how Christian theology in the modern age in America applies to politics is "Politics According to the Bible: A Comprehensive Resource for Understanding Modern Political Issues in Light of Scripture" by Wayne Grudem. You can argue about his intepretations and fudging of certain things a lot, but he's a hardcore theologian that writes textbook for the school of thought that dominates Seminaries and has for over a decade now.

Basically, if your pastor is under 40 or 45, odds are he was taught from textbooks written by Grudem or someone like him. From the pastors people absorb ideas; and thus an astonishingly small minority of people determine the long-term direction of a church-going nation.

Fascinating, no?