Another, social, arc of human history has been a gradual expansion of allegiance from immediate social groups to larger social groups, from families and tribes to states and nations, with the social inclusion and equal treatment before the law of more and more people previously marginalized or demonized as ‘the other’. This arc is largely an effect of the growing world population, recent social media, and the consequent coming into contact of previously isolated groups. It involves recognition of the common humanity of children, former slaves, women, of other ‘racial’ and ethnic groups, of sexual minorities, those who adhere to other ideologies and religions, and even other possibly intelligent species.
It’s fun to watch the religious reactionaries squirm, and make threats about civil disobedience or wholesale abandonment of the US for other shores, threats they will never follow-through on. They will get over it. It really is the arc of history, and progress, as has happened again and again, despite conservative resistance (Provisional Conclusion #8).
(If anyone is still obsessed by the so-called ‘traditional’ definition of marriage, see this cheeky Matt Baume video, linked at Huffington Post and many other sites, about the wide variety of historical ideas of marriage, especially from the Bible, which people today no longer endorse.)