The Atheist Perspective

Standards of marriage are like traffic laws: they are a complete invention of human cultures, and a society can do well with any it wants as long as they work (but, notably, can't do with none at all). Whether they will work depends on how well they integrate with other customs and institutions in that society. Most importantly, for polygamy to function, the rights and welfare of all parties must be equally protected, as they are now under modern monogamous marriage laws (or at least are supposed to be), and could conceivably be for polygamous marriage laws. Even today if a married man takes mistresses with the full knowledge and approval of his wife, they will be practicing a de facto polygamy that is perfectly legal under U.S. law, and yet his mistresses would have fewer rights and protections than his wife, and it is hard to fathom how it would be wrong to rectify that inequity in the law. The question of whether the arrangement makes any of their lives better or worse could only be answered on a case by case basis, just as with normal marriage. In short, polygamy as such is not immoral, but just like monogamy it can be used in immoral ways, and thus must be under a more informed and equitable regulation than it so far appears to be in those cultures that still practice it (very often, for example, the rights of women are not equal to men). As to whether it should be legalized in the United States, for example, in reality it already is, yet with less regulation than makes sense. A sensibly polity would treat the matter more rationally.

Nussbaum on Polygamy