The U.S. is a libertine state. They've got freedom up the wazoo. Gay marriage is just another Constitutional 'emanation' that SCOTUS will undoubtedly find come June.
To the erudite secular mind, the Bible was wrong about slavery (it should have never been), the Scriptures, too, are wrong about gay marriage (it's a freedom that should have always been). In addition to a girl required to marry her rapist and the stoning of an incorrigible child, the pagan will presume the Bible also requires an update on slavery and marriage. Just like Microsoft frequently requires patches and revisions.