Tradition! Tradition! Tradition! —“Tradition , ” Fiddler on the Roof (1964) On television, people marvel at the change. Marriage, that privileged heterosexual union, that millennia-long social institution said to be sanctified by the Christian God and his analogues, now seems to be going the way of other revered cultural traditions like slavery and human sacrifice; its terms are no longer so clearly defined. In the spring of 2013, eleven full-fledged nation-states permit...