ONE QUESTION for the Rev. Gene Robinson, the nation’s first openly gay Episcopal priest (now bishop) who cautions those of us opposed to same-sex marriage: “Don’t waste your time and energy defending marriage from something that doesn’t threaten it.” Doesn’t polygamy threaten marriage? Would the Rev. Robinson rise to defend polygamy, and perform wedding ceremonies for a man and two women, or a woman and two men (polyandry)? Because if same-sex marriage doesn’t threaten real marriage, why should polygamy? There’s not a dime’s worth of difference between the two institutions when you study them. This comparison is not intended to...