If gay marriage is cool, why not polygamy?

"Scarcity of women leads to a situation in which men with advantages — money, skills, education — will marry, but men without such advantages — poor, unskilled, illiterate — will not. A permanent subclass of bare branches [unmarriageable men] from the lowest socioeconomic classes is created. In China and India, for example, by the year 2020 bare branches will make up 12 to 15 percent of the young adult male population."
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Hudson and den Boer note that "bare branches are more likely than other males to turn to vice and violence." To get ahead, they "may turn to appropriation of resources, using force if necessary." Such men are ripe for recruitment by gangs, and in groups they "exhibit even more exaggerated risky and violent behavior." The result is "a significant increase in societal, and possibly intersocietal, violence."
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Hudson and den Boer suggest that societies become inherently unstable when sex ratios reach something like 120 males to 100 females: in other words, when one-sixth of men are surplus goods on the marriage market. The United States as a whole would reach that ratio if, for example, 5 percent of men took two wives, 3 percent took three wives, and 2 percent took four wives — numbers that are quite imaginable, if polygamy were legal for a while. In particular communities — inner cities, for example — polygamy could take a toll much more quickly. Even a handful of "Solomons" (high-status men taking multiple wives) could create brigades of new recruits for street gangs and drug lords, the last thing those communities need.
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The social dynamics of zero-sum marriage are ugly. In a polygamous world, boys could no longer grow up taking marriage for granted. Many would instead see marriage as a trophy in a sometimes brutal competition for wives. Losers would understandably burn with resentment, and most young men, even those who eventually won, would fear losing. Although much has been said about polygamy's inegalitarian implications for women who share a husband, the greater victims of inequality would be men who never become husbands.