Legalization of polygamy following legalization of gay marriage already happend in the Netherlands. It could happen here. Stanley Kurtz, in a must read article, identifies the fundamental connections between monogamy and democracy.
Alexis de Tocqueville, that great nineteenth-century student of America, pointed to the abolition of primogeniture (exclusive property inheritance by first-born sons) as the social key to American democracy. Once American children inherited equally, said Tocqueville, landed estates were dispersed, and the ethos of kin unity and hierarchy was replaced by a spirit of democratic equality. Yet America’s abolition of primogeniture was only the culmination of a process begun centuries earlier by the Christian Church. Muslim families arrange marriages to cousins and other kin, thereby reinforcing couples’ identification with family and tribe. But from the fourth century through the Middle Ages, the Church fought to protect individual choice in marriage, while prohibiting marriage between cousins and other relatives. That undercut social forms based on kinship and collective identity, ultimately leading to the triumph of democratic individualism in the West.
Yet the weakening or even disappearance of extended kinship groups from family life in the West poses a problem. If families aren’t going to be held together by collective honor, mutual obligation, and shared economic interest, how will they cohere? The answer is love. Exclusive affection for a unique individual is the structural foundation on which Western families are built. In polygamous societies, where marriages are arranged and wives and children live collectively, too much individualized love (for spouses or children) endangers group solidarity. Yet in a democratic society, individualized love is praised and cultivated as the foundation of family stability. So take your pick. You can have a love-based democratic culture of monogamy, or an authority-based hierarchical culture of polygamy. But–as the Reynolds Court knew–you can’t have both.