Increasingly, however, I come to the conclusion that it's problematic for the state to confer any additional privileges and rights on anyone just because they form a household. What about the singles, yo?
I think this raises an excellent point that's not often considered in the debate over gay marriage. Why should the state have anything to do with marriage in the first place? Is conferring special privileges on couples, whether gay or straight, fair to people who are single? Singles already have enough disadvantages as it is, and many of us are not single by choice.
Leaving aside arguments that venerate tradition (which are totally uncompelling to me), I think a case can be made that the state should promote marriage only to the extent that it is necessary for the perpetuation and well being of the species. As far as I know, there's evidence that children raised by two parents (whether of same or different sex) tend to fare better than those raised by only one. In other words, two moms are better than one (and roughly as good as two dads or one of each).
(As an aside, I don't think there's enough data about the effects of polygamous arrangements on the upbringing of children. To the extent that it seems unlikely to constitute a significant harm, however, polygamy is probably not something that should necessarily be absolutely prohibited.)
If two parents are better than one, then it's in the state's interest to increase the number of two-parent households. But let me stop here and make an important distinction. I said that the state has a reason to interfere in matters of family insofar as it promotes the continuation of the species and its well being. Perhaps a better distinction is to be made between maintaining population and improving the upbringing and education of children. While the latter would seem always to be desirable, the former may not be, because of concerns about overpopulation.
Thus, while the state has an interest in encouraging children that have already been born to be raised by (at least) two adults, it may not have an interest in increasing the number of children being born, especially if its population is growing too quickly.
What this means is that the legal benefits that come from marriage should not necessarily be the same in all times and places. In countries with declining populations, not only marriage, but reproduction is worth encouraging. If a nation is in danger of overpopulation, then contraception should be widely available and encouraged.
I think the key to a rational policy on marriage comes from making explicit the advantages of marriages for society. My position here is a version of libertarian paternalism. Ultimately, individuals should be able to choose to marry whomever they like, so long as its consensual. Nevertheless, while not forcing anyone to do anything, it's acceptable to create mild incentives and disincentives to encourage people to make one choice over another. If a cohabiting couple with children would be more likely to stay together if married, it's not inappropriate for the state to offer some special privileges for obtaining the legal status.
In short, I disagree with IOZ because I can see a point to encouraging marriage in some circumstances. As I've suggested, I think marriage should be allowable between any parties which are capable of consenting (so this would extend beyond gay marriage to include things like polygamy and, eventually, human-robot marriages). I also favor the policy of separating the civil institution from the religious one. Religions should be allowed to set whatever requirements for their marriage ceremonies that they please, but any consenting couple (or group) should be allowed to obtain a civil marriage and the legal benefits it brings.
(I can really see myself now as a kind of left-libertarian paternalist. I like the idea of using a wider range of incentives than merely criminal punishment to influence the behavior of a population, without actively coercing individuals. In every case, we should use science and other knowledge to determine what is good for people as individuals and as a whole, and then incentivize choices to make self-interest coincide to a greater extent with the common good.)
(Sorry if this is a bit rambling. I wanted to record these thoughts before I forgot about them. I definitely appreciate any feedback about the arguments I put forth here.)