I do not expect this to be an easy piece for anyone to read. I do not intend to pull punches, and I expect most readers to be offended at times. I expect everyone to wonder "how can he think that?" at various points. And so I will do my best to be clear when offering what I think are revolutionary critiques of this issue from various angles.
My view is simply that the grass-roots is more able to sort out complex moral issues than the state is. Neighborhoods, community projects, religious groups, etc. do better here than the large organizations. As per previous posts, I think subsidiarity requires a grant of general autonomy to both individuals and organizations. I think a state, by being truly neutral and saying "maintaining culture isn't my concern" can encourage a healthy discussion on the cultural level. I also think that concerns about normalizing homosexuality are overblown but for a decidedly unpopular reason: I see the difference between the left and right here as between versions of aversion to homosexuality, not between acceptance or rejection of the idea.
Some readers will fear I have read too much Derrida, and others will be outraged that I would dare to deconstruct the "left" on this issue. Deconstruction, I am sure, is seen as a weapon to be used against everyone else, but here I think it is especially useful.
Is Same-Sex Marriage Sustainable? An Historical Perspective
In virtually all cultures at virtually all times in history, marriage is a complex institution which combines sexual restraint with procreation and care for (and enculturation of) children. Marriages serve many secondary functions including, but not limited to, solidifying family alliances, caring for the elderly, providing economic and political foundations and more. The specific functions vary from culture to culture. However, in most cases, procreation is generally expected to be an aspect of marriage. Procreative unions are more universally ritualized than death and disposing of human remains.
There are of course two notable exceptions, which were Rome and modern, Western civilization. In both these cases, there have been a great deal of controversy on the matter, and there is some controversy as to whether some of our earliest surviving Christian liturgy are related to same-sex marriage or something related. However, as I will illustrate below, in every case same-sex marriage has culturally been discouraged even where recognized, and modern society's passive-aggressive "acceptance" of homosexuality is no different.
Of course issues of homosexuality and same-sex marriage are separable. Homosexual sex is woven into many cultures in various ways, but marriage is usually procreative and separate. This post does not address sinfulness or lack thereof regarding homosexual sex, but only marriage and society.
Society, Cicero said, has certain functional requirements without which we would not be able to live together in communities. In this way, he has largely laid out the basis for functionalism in sociology and anthropology. We can therefore derive some insight into the roles marriage plays in various cultures by looking at the inherent social problems it often solves, and compare with those societies where same-sex marriage is even thinkable in order to understand the nature, and sustainability limits, of the practice.
As we live life, we go from being economically unproductive children to being economically productive in various ways in different stages of life, to being elderly and again economically unproductive. Support for the elderly then becomes a key social issue. In complex societies that can afford it, the wealthy may entertain an independent retirement, while in simpler societies tend to have communal support, particularly through the family, for elderly. The childless end up with fewer options, which is one reason why procreation is very important in these societies.
In Rome, the upper class could contemplate an "independent retirement" where a man could retire to his rural estates and be supported by slaves. This meant that for upper classes of society, there was not a mutual support requirement of retired people and their children. When same-sex marriage did come up, it was a big deal and broke taboos, but it did not threaten the most important functions of society regarding children and the retired. Similarly today in the US and Europe retirement is independent, paid for out of pensions and savings, and therefore as in Rome, we do not have the same grandparents caring for the grandchildren that exists in similar societies. Moreover this is not limited to the upper classes but is intended to be good for everybody (I am not sure it is, but that is another topic).
Our society is capable of this not because we are "better" than we were a thousand years ago but because our social complexity is subsidized by the burning of fossil fuels. Thus there is every question of what will happen as energy prices continue to rise and whether this forces a retraditionalization of the family. We are already seeing a rise in multi-generational households and there is no reason to think that will reverse anytime soon.
Another point that cannot be overstated is the need to separate a discussion of sexual morality from marriage in historical discussions. In many historical societies, extramarital sex for various reasons was condoned and even supported. Many traditional societies had traditions of wife-loaning and wife-swapping, but always the husband was the lawful father. Moreover polygyny and polyandry both also count in this area even though the latter poses paternity issues the former does not. Van Gennep's examples show the paternity issue in polyandrous societies as often solved ceremonially, showing the point that everwhere mothers are mothers by virtue of giving birth but fathers are fathers by virtue of social recognition (this is no different in the US today). Wife-swapping as well as pederasty formed the political backbone of ancient Sparta, and Aristotle wrote on restrictions on pederasty to prevent it from being socially problematic as well. Van Gennep talks a bit more about pederasty, and also temple prostitution, in a cross-cultural perspective too. That such societies that differ so much on questions of sexual morality maintain a largely consistent core of marriage (focusing on procreation) tells us something important, that this is not about sexual morality per se.
Thus I think we can show that the ability even to have this conversation is a bit of an economic luxury bought by the very burning of the fossil fuels which are causing damage to our environment. I therefore don't see it as a particularly important question or one that the state needs to concern itself with. Support the family, nourish the family, take care of the other problems, and this issue by the time you get to it will have changed beyond recognition if it is an issue at all.
The Gay Rights Movement of Today: Towards a Kinder, Gentler, More Effective Homophobia
There is often a general fear that by recognizing same-sex marriage that the state is normalizing what should not be normal. This is perhaps the most common argument against same-sex marriage but it misses the general social dynamics and the pervasiveness of structures which discourage homosexuality in our culture.
In this section I will use the term "homophobia" to refer to the various social structures which discourage homosexual sex in our culture. It is my view that the "acceptance" of gays and lesbians in modern society is very passive-aggressive and done in a way which ensures that this is even more taboo than it was when the taboo was overt.
In the United States today, marriage is portrayed fundamentally as heterosexual and monogamous. This thread runs through children's toys and movies to bridal magazines, and the like. The model of normality is very strictly around heterosexuality. One can imagine the outrage and controversy that a Barby and Kendra dyke Barby playset would create even among people generally supportive of the gay rights in the US. This would be seen as sexual in a way that Barbie and Ken are not.
We might see full normalization of homosexual relationships as the point where such toys are there, and where same-sex relationships are portrayed in Disney movies for little girls, and where this is all entirely uncontroversial. Yet according to the general rhetoric of the gay rights movement, this can never happen. Gays and Lesbians, we are assured, are born that way, and will always be a tiny minority. However, what this does, in the context with the aggressively heterosexual models of normality is send a message that homosexuality is basically a birth defect. The born that way thesis, far from ending homophobia, merely transforms it. It sends the message to every young man and woman, "you aren't born that way, are you?" In this regard it is not really different from the Calvinist doctrine of predestination, in saying that salvation is inherently reserved for some individuals and ensuring that people demonstrate that they are in the favored group.
There is a fundamental difference between seeing homosexuality as "bad behavior" and an "inborn trait, rare, and at odds with society's definition of normal." The first category is far more forgiving of experimentation than the latter is, and the latter thus creates a paradoxically stronger taboo regarding homosexual sex. After all if it is just bad behavior then self improvement can make you stop, but if it is an inborn abnormality, then some things are just better left unexplored.
The gay rights movement then pushes for acceptance of homosexuals, but does so in a framework which labels homosexuals as fundamentally other, and therefore holds them out as an example of how not to be a member of the preferred social group. In the social context, then it is not about normalizing homosexuality but in deriving a little comfort for those outside while actually tightening up the taboos on the subject. The rhetoric cannot be separated from its social context.
This is not really all that different from how the classical modern feminism sought to merely flatten gender roles and allow women to be men too, and in the end created a form of feminism which often works against the interests of women particularly in the medical context.
A Look to a Simpler Future
In the future our fossil fuels will become more and more expensive and energy prices will follow. This will create a huge burden on the modern economy, and one which mere fuel efficiency standards in automobiles will be unable to address. As we are forced to look to do more with less, then the question becomes how do we structure our families to do this? Do we put money into our children's education instead of our retirement in the hopes that they will be able to graduate with less debt, and yet we will be interdependent in retirement?
But beyond this, what happens when we cannot sustain the level of complexity we have today? What happens when the sperm bank is no longer an option for lesbian couples wanting to conceive? The very options that make same-sex marriage possible today may not be there in the future, namely the separation of sexuality from reproduction, and the ability for most people to have an independent retirement.
 Grimes, Ronald. "Deeply Into the Bone: Re-Inventing Rites of Passage"
 Cicero, "The Republic"
 van Gennep, Arnold. "The Rites of Passage"
 Plutarch. "The Life of Lycurgas"
 Compare to the masculine-normal model of the human body in medicine, see Davis-Floyd, Robbie. "Birth as an American Rite of Passage."
 Davis-Floyd, Robbie. "Birth as an American Rite of Passage."