ANTHONY KENNEDY, in his majority ruling legalising same-sex marriage nationwide, tried to allay the concern that polygamy would be next. John Roberts, in his dissent, said he couldn't see a principled way of opening the door to same-sex couples without also letting polygamists through. Who's right? Is legal polygamy next? Should it be? Jonathan Rauch, who wrote this newspaper's ahead-of-the-curve 1996 cover piece advocating same-sex marriage, has plenty of experience in keeping people off this slippery slope. According to Mr Rauch, the problem with a man marrying two women (we don't see many real-world examples of polyandry, a women with...