There are two approaches to polygamy. One is obviously rooted in religion. Religious polygamy has always been in the form of one man, two or more women. Granted that the 1st amendment guarantees the right of practicing any and every religion, the line is drawn where such practices harm others. Since in religious polygamy women hardly have a choice in such a marriage, that obviously goes against their freedom and consent, again upheld by the same constitution. If a mechanism is used to insure that women are entering such marriages of their own free will, I think such unions should be allowed. However, I don't think the government would be too keen to extend the benefits of marriage to the 2nd and 3rd wives. I suppose if that would be the case, many women would think twice before getting involved in a harem way of life. Young boys with no chance of getting a mate is another bad side effect of such unions.

The 2nd approach to polygamy, might sneak in sooner than we think with the overpopulation of the earth and shortage of resources. It's a communist approach. People who realize they can't afford to work and pay for all the childcare expenses might join forces. Imagine four people, three working and bringing home the dough, one staying and taking care of the children. They could provide better for their kids. Just for a moment take the sex or swinging out of the equation and just consider the economics of such a relationship. As mentioned in the previous approach, the government recognizing such clusters of society as marriages is too much to expect.