Are you sure you're not reading your own domestic preference into his position? Very hard for me to see Rawls arguing against polygamy just as long as all parties consented — JerseyFlight
My response is a bit long, but I think it's worth it to try and make the case as convincingly as I can... in the space of about an hour of my day.
It's possible that I'm over-reading Rawls as you suggest, but here are some references that I think support my reading of Rawls in regard to marriage (potential objections addressed below).
"For us the primary subject of justice is the basic structure of society, or more exactly, the way in which the major social institutions distribute fundamental rights and duties and determine the division of advantages from social cooperation. By major institutions I understand the political constitution and the principal economic and social arrangements. Thus the legal protection of freedom of thought and liberty of conscience, competitive markets, private property in the means of production, and the monogamous family are examples of major social institutions. Taken together as one scheme, the major institutions define men's rights and duties and influence their life prospects, what they can expect to be and how well they can hope to do." — Rawls, A Theory of Justice (rev. ed.), p. 6
"Furthermore, the principle of fair opportunity can be only imperfectly carried out, at least as long as some form of the family exists. The extent to which natural capacities develop and reach fruition is affected by all kinds of social conditions and class attitudes. Even the willingness to make an effort, to try, and so to be deserving in the ordinary sense is itself dependent upon happy family and social circumstances. It is impossible in practice to secure equal chances of achievement and culture for those similarly endowed, and therefore we may want to adopt a principle which recognizes this fact and also mitigates the arbitrary effects of the natural lottery itself." — p. 64
Let me say a few things about each quote. Regarding the first quote, obviously this quote provides the most potential difficulty as the term "monogamous family" would seem to rule out an open, polygamous marriage. Furthermore, one might also question the link between family and the more specific state of marriage.
Answering this second problem first, it seems obvious that by "monogamous family" Rawls simply means "monogamous marriage." My reasons for this are three-fold. First, the idea of a "monogamous family" (as opposed to marriage) is prima facie nonsensical. Second, let's assume that the concept of a "monogamous family" does make sense if we stipulate a certain definition. This seems unlikely since Rawls never stipulates a definition. But whatever unusual sense we stipulate will likely end up being insignificant to the type of privileges that Rawls has in mind and which social scientists typically think result from stable marriages (just do a google search if you aren't familiar with this data). In other words, it would be strange, not to mention implausible, that Rawls thinks certain privileges result from monogamous families-but-not-marriages where this means brothers and sisters who don't have any adopted brothers or sisters or form close bonds with non-blood relatives that might be considered colloquially "blood brothers." And the third reason follows from this and really just further strengthens it, which is that Rawls later speaks positively of the principle of fraternity (I won't take the time to look up the reference here unless someone wants it).
Now regarding the first and more important objection, my answer has to rely more on considerations more external to the text. First, at the time Rawls was writing (even in the revised edition in 99) I don't think there was enough data from the social science on non-monogamous stable marriage. That is, most of the data for comparison with monogamous marriage would have been of relationships that involved one man and one woman who were unmarried. There wouldn't have been a large enough sample size of same-sex marriages or open, polyamorist marriages (there still is not much for the former, is my understanding, and even less for the latter, since polyamory is still illegal in the U.S.). Furthermore, when Rawls was writing (again this is largely true in 99 as well) non-monogamous, non-heterosexual marriages weren't even on most people's radars. (Recall that Clinton signed DOMA into law and Obama only supported civil unions in his first term.) So most likely Rawls isn't trying to say anything about polyamorous or open marriages or even same-sex marriages. Rawls is probably just thinking something like the following: "when people get married and stay married this has certain privileges over against when people don't get married or don't stay married."
But today we have a much more expanded view of which types of long-term relationships and marriages are possible and permissible. This is why, to my knowledge, marriage doesn't feature as a consideration in his original position. It likely just wouldn't have occurred to Rawls for monogamous marriage to feature in the thought experiment since, in his context, of course society has to have marriage and the only alternative is something like hook-ups or divorce. But there's no reason that we shouldn't consider marriage arrangements that weren't on his radar or hardly anyone else's at the time.
Next, turning to the second quote. The last sentence brought to mind that someone might object to my inclusion of marriage arrangements in the original position because those things which Rawls considers in the thought experiment are already
meant to address the sorts of inequalities which result from the basic structure of society. In other words, Pfhorrest's objection might turn out to be correct. Since the principles of justice that Rawls arrives at through his thought experiment balance out the inequities of the basic structure, it doesn't matter what sorts of marriage arrangements we do or do not have.
However, I don't think this is a successful objection because Rawls wants (and his representative people want) to arrive at the most just
society and not simply a
just society. Thus, connecting with my answer to the first objection of the first quote, that Rawls arrived at a theory of justice which doesn't prefer any arrangement of marriage and which he thought would lead to the most just society doesn't mean that we, with a broader landscape for social and relational arrangements, must limit ourselves to Rawls's arrangement. Rawls would have agreed with this himself, since at one point he mentions that we are allowed to take data about what works or doesn't work, say, economically, into account and (I don't remember if he says this explicitly) that must include new data from future generations or experience. (Will look up the source if someone wants it.)
Finally, I'll reiterate that it's possible that my entire project here is naively off-track since there's an obvious principle or thing Rawls says which renders my argument irrelevant or answered. I've gone through the trouble of writing the above simply on the assumption that I'm on the right track.
P.S. And of course as Pfhorrest brings to our attention, if polygamy is not unjust then my further point about challenging the original position is moot. Right now, I'm more interested in whether the more immediate claim goes through.