In a new century dominated by terrorism and recession, few would deny two big bright spots: the election of an African-American president and the expansion of gay civil rights. The first arrived nearly 150 years after the Civil War. The second happened with the speed of a fever dream. The modern gay-rights movement only got going in 1969, after the Stonewall riots. Now a dozen states have legalized same-sex marriage, a concept unknown in political discourse a mere quarter-century ago. More astounding is the likelihood that a conservative-leaning Supreme Court will expand those marital rights, however incompletely, next month—it took more than a century after the Emancipation Proclamation to end all bans on interracial marriage.