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But I don't think there would be a male-surplus problem in a Westernized country that adopted polygamy. What would really happen is that men inclined to have more than two spouses would likely import a good number of Second- and Third-World participants.
It's also a mistake to consider solely the effects of a change from monogamy to polygamy by itself. If the switch were combined with a legalization of prostitution, this would reduce the excess demand for wives. Cf. Posner on the subject of sexual marketplace competition.
The reason why polygamy isn't considered woman-friendly is because of other legal rules. How does one resolve questions of divorce? Community property? Child custody? There's no polygamous society that answers these questions in a manner that favors women. Other than prenuptial contracting, there's no legal regime that answers these questions in favor of wives that makes polygamy economically attractive...