As the national marriage rate remained stable, the marriage rates in states that redefined marriage fell at least 5 percent. This is perfectly logical. If marriage is simply a matter of romance between consenting adults, some persons, content simply to cohabit, will not bother to get married. But cohabitation isn’t as stable as marriage and…a drop in the marriage rate of only 5 percent would mean over a million fewer women marrying.
In the Netherlands—the first country to redefine marriage as a genderless institution—marriage rates for young women dropped an additional 5 percent over the rate at which they were already declining. That is, same-sex marriage is a symptom of a collapsing marriage culture, and it then becomes a cause of further and more rapid decline.
Ryan T. Anderson, “Truth Overruled: The Future of Marriage and Religious Freedom,” pg.159-160