The arc of the moral universe is long, said abolitionist Theodore Parker. “My eye reaches but little ways. . . . And from what I see I am sure it bends toward justice.” But few advocates of social change — in Parker’s time or ours — act as though justice really is inevitable. They try to hasten it. So it is for advocates of marriage equality, who, against once-impossible odds, have emerged from the political and legal wilderness. Nine states and the District, encompassing about 15 percent of the U.S. population, have legalized gay marriage. This includes three states whose voters approved it on Nov. 6. Though it was opposed by a clear majority, 57 percent to 35 percent, in a 2001 Pew Research Center poll, gay marriage now enjoys a 48 percent plurality. And the Supreme Court has agreed to consider overturning both the federal Defense of Marriage Act and state bans on gay marriage, raising the prospect of a national victory — but also risks. The court could reject the advocates’ arguments, setting a negative precedent. Or it could accept them — triggering a backlash even sharper than the pro-death-penalty reaction to the court’s attempt to strike down 40 state capital punishment laws in 1972.