Senator Sam Brownback goes even farther than Stanley Kurtz, offering as a reason for supporting the Federal Marriage Amendment that
The experience of Europe also shows that the decline of the institution of marriage goes hand in hand with a decline in married fertility, and a corresponding decline in population. Because of the birth dearth in Europe, many countries find themselves faced with the prospect of aging (soon to be shrinking) populations and an impending collapse of their social-welfare systems because of a declining ratio of workers to retirees.
towhich, of course, the response is: there’s no variation on the dependent variable. All rich developed societies undergo a decline in fertility, regardless of the state of their marriage laws. The United States, Japan, Italy, Ireland, and the Netherlands have very different marriage laws and public cultures regarding sexuality and family life. All have seen very sharp declines in fertility, to levels below replacement. In the absence of immigration, all will see their populations shrink. (Not all of their populations will in fact shrink, as some of those countries have substantial immigration.) Yes, even Ireland, which has the strictest divorce laws of any developed country, no legal abortion, and certainly no same-sex marriage, has below-replacement fertility– and its fertility was falling fast even before divorce was legalized in the ’90s.
So, yes, rich developed countries that have embraced liberal attitudes on homosexuality and divorce and cohabitation have experienced fertility decline. So have all the other rich developed countries. There’s not only no causal argument here; there’s no correlation.