[Note: This post was requested by Laura, one of our big-spendin' financial supporters. While making a donation through this site will not guarantee that we'll read/write about something you request, greasin' the wheels won't hurt.]
I've used the gay marriage issue as an example of a prototypical example of progressive morality: something that we should have realized (as a society) was perfectly fine a priori, but which it's taken us a lot of growing up to accept. A good moral theory, of course, should allow us to call these cases far before they're trendy, and feminist philosopher Elizabeth Brake argues in Minimizing Marriage: Marriage, Morality, and the Lawthat (this quote is from an extract from her book):
...Marriage should be demoralized—that it does not have a sui generis moral status or a transformative moral power. ...The great social and legal importance accorded marriage and marriage-like relationships is unjustified, and that this privilege harms, sometimes unjustly, those not oriented towards monogamous, central relationships. Those harmed include members of multiple significant overlapping friendships such as adult care networks or urban tribes, the asexual and solitudinous, and the polyamorous. ...A truly politically liberal law of marriage would expand the legal category of marriage in surprising ways, minimizing special restrictions on entry, exit, and what transpires between.
To get an idea of her position, watch this debate on Philosophy TV with Simon May. The two of them rehearse the arguments surrounding gay marriage (which they both support) for the first 17 minutes before getting around to Brake's thesis: she wants to extend the state neutrality between heterosexual and homosexual marriage to neutrality with regard to any consensual, adult relationships. "While there may be special goods in caring relationships, they do not depend on marriage—and, indeed, the special value attributed to marriage has penalized caring relationships that fail to fit the marital norm." As May characterizes Brake's position, the state is just not the optimal organization to figure out and thus legislate what human flourishing entails.
Around 20 minutes in, they deal with the polygamy issue: If it's entered into by consenting adults, then it's not intrinsically unjust. Yes, there can be tragically unequal power relationships within such relationships, but the same problem plagues heterosexual monogamy. At 30 minutes in, she acknowledges the practical problems in polygamous communities but thinks that the solution is to correct the abuse, not to outlaw the practice.
So why have the state involved at all? Well, because there are legal issues to deal with having to do with property, medical decisions, etc. The state can't avoid being involved so long as there is legislation in such areas, e.g. as long as transferable private property exists in the society. (May brings this up around 34 minutes in.) Brake's concept of "minimal marriage" is articulated around 41 minutes in.
So what's my take? I'm open to Brake's thesis, and think that public policy needs to proceed in accordance with the harm principle: allow anything that doesn't systematically and unquestionably cause harm. As a practical matter, we always start legislating where we are now, so it's not a matter of suddenly allowing everything and then have legislative battles about which barriers to put up, but a matter of being more open-minded and accepting of testimony as to what situations have already been pursued and are working out. So, for polygamous relationships, let a community essentially apply for legal recognition, have some studies and things re. what potential new problems can arise in such a circumstance and take measures to address them, but if the situation appears on close study to be copacetic, then grant the rights, not just to those individuals applying, but to anyone else applying for the same sort of recognition.
So what determines "same sort" as far as relationships are concerned? Lots of complications come in here: for instance, does it matter if the relationship is sexual? Marriages don't get disqualified if the participants aren't sexual (e.g. if one is injured). Could two people enter into a partnership that in fact rules out sexuality (e.g. family members that want power of attorney, etc. over each other and want the rest of the benefits)? Need the participants even have met face to face at all (e.g. Internet "relationships")?
Social engineering in a democracy works such that if people aren't complaining about it, then no legislative will is there, so nothing does or should get passed. This faces the endemic problem that even a small minority should be given a decent shake even if their views are too weird for consideration by the wider populace. This is where the judiciary is supposed to step in and adjudicate on the specifics, as analyzed by the bureaucrats. It's not as if we need legislative action to permit specific instances of speech once we have free speech in pace. So if legislation were proposed granting, in general, rights to any type of relationship that meets some abstract standard, I'd likely be for it.