What happened? A lot of things did, but the clamor that arose in the 1960s for individual autonomy (a/k/a "What I Say Goes!") is a convenient starting point for examination.
An intellectually sloppy and devitalized era that valued Self more than it esteemed some-ol'-God-out-there-in-space couldn't think of a single reason that people who clamored for "justice" shouldn't have it. Same-sex marriage came under that heading once the political power of the gay lobby waxed and that of the religious community waned. Bring on the politicians! -- the vote-seekers, the money-raisers, the Cuomos, the New York lawmakers in whose hungry mouths butter refused to melt as they pretended to reinvent marriage.
Thus, for the first time in history, two institutions, both known to their practitioners as marriage, lie oddly alongside each other: in New York and a growing if still gratifyingly small number of states. The institution that brings unity out of difference is of course the real one, rooted in Nature. The other is phonier than a Bernie Madoff cross-my-heart-and-hope-to-die.
In the end, reality trumps fantasy, which is why New York's experiment in moral Madoff-ism will shrivel and die. But how long before it does? And after what number of disasters in the lives of real human beings looking to their culture for guidance?
Playing with real people's real lives and offering cheap and transitory satisfactions for the sake of political gain is more the mark of fascism than of American democracy. Or at least it once was.