Polygamy

This is essentially a libertarian vs. egalitarian issue.

Becker's argument for polygamy:

...[Y]et isn't it offensively patronizing to women to believe they cannot make their own decisions about whether to enter into marriages that contain other wives? We do not offer men any special protections against the "wiles" of women, so why do women need such protection? Indeed, I believe that in marital decisions women are more thoughtful and far-sighted than men, partly because marriage has meant much more to women than to men.

The claim that polygyny is unfair to women is strange since polygyny increases the demand for women as spouses in the same way that polyandry would increase the demand for men. If men were to take multiple wives, that increases the overall competition for women compared to a situation where each man can have at most one wife.

Even though women as a group would gain from allowing polygyny, and men as a group would be hurt, not all members of each group would be affected in the same way. Men who do not have much to offer women would be more likely than under monogamy to remain unmarried, at least until they become older and wealthier, or more matured. Similarly, educated and otherwise attractive women who have a lot to offer might suffer if they have to face competition from several women who individually have less to offer, but collectively can offer as much or more...

And here is Posner's counter arguement:

My view is that polygamy would impose substantial social costs in a modern Western-type society that probably would not be offset by the benefits to the parties to polygamous marriages. Especially given the large disparities in wealth in the United States, legalizing polygamy would enable wealthy men to have multiple wives, even harems, which would reduce the supply of women to men of lower incomes and thus aggravate inequality. The resulting shortage of women would lead to queuing, and thus to a high age of marriage for men, which in turn would increase the demand for prostitution. Moreover, intense competition for women would lower the age of marriage for women, which would be likely to result in less investment by them in education (because household production is a substitute for market production) and therefore reduce women's market output.

Because polygamy is illegal everywhere in the United States, few Americans think of it as an option. If it were made accepectable by being legalized, who knows? There are 400 American billionaires, and several million Americans with a net worth of at least $6 million. Nor, with most women working, is it obvious that a man would have to be wealthy in order to attract multiple wives, though presumably men who wanted to be polygamists would have to be able to offer some financial inducements, since most women would prefer to be a man's only wife. As more and more men attempted to become polygamists, the "price" they would have to pay for a wife would rise, so polygamy would be a distinctly minority institution. But it would not necessarily be trivial in size or harmless in its social consequences, which would be likely to exceed those of homosexual marriage...

I think Posner opposes polygamy not only from a fairness point of view, but also from that of overall social welfare, which makes more sense to me by the way. Here are the original blogs from Beck and Posner.