The problem with an anti-traditionalist view is that it fundamentally isolates people from eachother, destroys all sense of place, and thus stunts human flourishing. There is a rising traditionalist movement both among the left and right of American politics, and I place my hope there instead of in the anti-traditionalist progressive wing.
The Traditionalist Defence of Same-Sex Marriage and Gay Rights Hostility
In the primary traditionalist view, marriage is about children as it is universally across cultures and history. The basic issue anthropologically is that men and women are differently situated regarding reproduction and so societies are generally forced to choose between women bearing sole responsibility for childraising and creating a social and legal framework for sharing such support between men and women. That social and legal framework is marriage and is more universally ritualized than is saying farewell to the dead.
This view is echoed in Aristotle's "Politics" where he suggests that there are three layers to society, namely the individual, the family household, and the polis. The household's main role is that of creating children and passing culture on to them so they can form households that are within the union of the polis. Of course in addition, the household is an economic unit, and one which supports the elderly in most societies and times, but all of this is put in the primary service of the raising of children.
The family household is the bedrock of the state. It provides a stable foundation for raising children and this also provides the basic unit of grass-roots culture of all sorts. Strong families then produce productive citizens, and this is why Cicero said (in "The Republic") that one should not turn one's parents in to the state for crimes because the state needs strong families more than it needs criminal justice.
It is worth noting of course that neither Aristotle nor Cicero likely had any problem with male-male sex per se, and it isn't clear what Greek attitudes were towards female-female sex were, though there is some evidence for widespread occurrences of this in religious contexts in vase paintings and the fact that the religious organizations that gave rise to Sappho's erotic poetry were also found throughout the Greek world. Of course the Greek world was full of rules regarding who could have sex with whom and what sorts of acts were acceptable, as are all cultures.
From this perspective then, marriage creates a family household with characteristics which make it uniquely qualified for childrearing. These include essentially a form of corporate existence which can outlast the death of any single member without necessarily there being a loss of property, custodial rights and responsibilities to children, and the like. But children are the future and they are the focus.
The decline of the American family then occurs through a series of stages including the rise of the ideal of an independent retirement, the shift in voting from per household to per adult. As time goes on however, we have in my lifetime supplanted more and more of the traditional family structures with institutions, changing from neighborhoods and children playing together to factory farm day-care/preschool/public school systems chained together with ever-lengthening expectations of years that children shall go through that system. And we expect schools to play ever-increasing roles in culture war issues and many of the huge battles over abstinence-only sex ed, and portraying homosexuality as normal in public schools is a result of this. Add to this ever-increasing social services supplanting parent roles so both parents can work and the elderly can have an independent, if lonely, retirement in the name of not being a burden to anyone. This decline of the family and replacing it with government institutions is very concerning and fortunately there is a small, growing traditionalist movement which is seeking to reverse this to some extent. This includes a nacient domesticity movement as well as a general increase in multi-generational households particularly as immigrants come to the US.
From this perspective, same-sex marriage is not necessarily a bad thing. It provides, for children who are in de facto same-sex households anyway, the same legal protections regarding continuity of their environment that those children have in opposite-sex households. If marriage is about the children then the question of same-sex marriage needs to be about the impacts on the children not the impacts on the parents, and if marriage enables better support of the children, then we shouldn't disadvantage the children just because they are raised in a household where both main contributors are the same sex.
Such an argument has a potential to broaden the appeal of same-sex marriage as a cause but every time I have suggested it, I find myself under attack mostly from the very advocates of the cause I am suggesting could be furthered there. It is hard to understand why advocates for a position would be actively hostile to people reaching out to others on behalf of that position, but I think the issue is that it is easy to see this as dangerously close, from their perspective, to the idea that marriage is about procreation. I don't think it is a problem to suggest that marriage is about procreation cross-culturally, though, or that we are in a relatively unique period of history where a combination of factors are temporarily weakening that interest including the fact that we have had a huge social subsidy through fossil fuels which has largely fueled the modern age.
I think that these are almost undeniably true, and that as energy prices continue to rise many things we take for granted including the potential for an independent retirement for most people. Our consumption of medical technology today is also unsustainable, as is the apparent middle-class boom in wealth caused by housing prices continuing to rise. The very things which enable us to see this as an issue today are not guaranteed to be around far into the future. Consequently I think there is some wisdom in avoiding the arrogance which suggests that all previous generations have done things wrong.
The fact is, tradition is dynamic. Aspects of tradition can be repurposed and traditional rites can change greatly in a short amount of time. The lack of a traditional framework in modern culture has a great deal of cost in terms of mental health. Consequently one of the promises of the movement to retraditionalize society and in particular the family is that this improves sustainability long-run. Tradition thus doesn't necessarily mandate certain viewpoints, but instead provides a narrow rhetorical framework for arguing about them, for example making the children the focus of any debate on same-sex marriage. The fact though is that while this is constraining it offers a sense of place and stability that is lost as tradition atrophies. One major reason for the New Age movement may be a reaction to the loss of our vital traditions.
The failure to accept that people may embrace a traditionalist view of family and yet support same-sex marriage means that traditionalists such as myself find ourselves struggling on one hand for the idea that children need the support that marriage offers in same-sex households and also the fact that the proponents of same-sex marriage believe that tradition is what holds us back from social "progress." But it is a funny sort of progress which uproots people from their towns, and weakens families, robbing people of a sense of place in the name of "freedom," and it is a funny sort of freedom that does not include a right to one's traditions. I have even heard opponents to my views suggest that the sustainability issue isn't real because we can solve it by building large numbers of nuclear reactors. Great, supporting same-sex marriage requires ensuring many more Chernobyls and Fukushimas..... If that is the price, no thanks.
But I think we are offered here a false choice. Another option is that we focus on reconnecting the family with the raising of children and the care of the elderly and worry less about the specific structures of other people's households in that area, and recognize that same-sex marriage may well be a way to help do this if the context is right. The major problem is that non-traditionalists (which comprise the city folk, both right and left) tend to be hostile towards the role of marriage in raising kids. There are a number of reasons for this, some more legitimate than others, but this is the backdrop under which this dialog occurs.
Saving marriage in its traditionalist sense does not necessarily mean banning same-sex marriage. It does however mean that adult children should live near their parents to the extent possible, and that mutual support and assistance in raising future generations should be offered, but fighting for this means fighting against the tide on both sides of the political spectrum.
The Gay Rights Movement as Homophobic
The second major problem I see with the gay rights movement is that it is fundamentally homophobic insofar as gays and lesbians are treated as fundamentally other. This sets up a sort of passive-aggressive acceptance of homosexuals on the basis that they will never quite match society's definitions of "normal." The idea is that gays and lesbians must have equal rights because they do not conform to societal models of normality. Because they are fundamentally other, they cannot be normalized, just accepted. We will never see a market for a dyke barbie playset because, we are assured, homosexuals are a small minority for biological reasons.
This is not the only area of course where we see this sort of passive-aggressive acceptance of "the other." It is common in race and gender as well. For example, there is a significant gender disparity in medical textbooks on figures representing health and disease, with male figures more often representing health and female figures more often representing disease. One can find similar racial disparities in many areas. For example, Time Magazine darkened OJ Simpson's mug shot in the 1994 issue focusing on the murder of Nicole Simpson and OJ's arrest. The subtle standards of normality are brutally pervasive and all rhetoric must be read in their context.
That context of normality in modern culture is defined by Disney movies for little girls, by Barbie and Ken, by romance movies generally, and by bridal magazines. There are industries upon industries which make money essentially selling heterosexuality to young girls and young women. Therefore heterosexuality is normal, and people who deviate from that are not. The cultural argument then boils down to "are they bad? Or is it that they can't help it?" The basic assumptions accepted on both sides of the debate are deeply homophobic and seek to constraint as many people as possible to a life of exclusive heterosexuality, and in this context "gays are born that way" carries with it a subtext of "you aren't born that way, are you?"
Of course the reason is that once you reject this false choice, many more unpleasant questions start surfacing and the issue becomes very difficult to contain. The idea that homosexual sex might actually be normal within limits for most people forces back the questions relating to marriage, why we fight against it so hard, and so forth. There are however growing numbers of people (myself included) who reject sexual orientation labels because we see them as social control mechanisms, and this brings us back to the fundamental questions of same-sex marriage but from a viewpoint so far unlike what goes on in political discourse that it just can't be reconciled with either major camp.
It is as if our society presents us with two boxes, marked "gay" and "straight" and tells us to get in one and stay in it. If we look around there is a small box hidden behind those two labelled "Bi" but anyone who goes in that box gets treated with suspicion by everyone else. But the natural condition is not to choose any of the boxes. Many societies have woven homosexual sex through their societal structures in various ways and therefore I conclude that virtually everyone is capable of having fulfilling sexual relationships with people both of the same sex and opposite. This may feel intuitively wrong in our culture, but we cannot separate the industries and social forces which construct our taboos from those feelings.
I contend that until one moves beyond the general hostility towards homosexuality in our culture, a hostility which deeply pervades even the gay rights movement, we cannot really discuss on even terms the question of same-sex marriage, and once we do the issue will be so heavily transformed we will no longer be able to recognize it. I think, however, if you separate the traditionalist framing of the issue from hostility towards homosexuality it makes more sense than the anti-traditionalist view. The only question left is which direction to take it, while focusing on rebuilding the family as the bedrock cultural institution and taking back authority from the state to vest in the family.
Fleshing out a view of human sexuality which works from a cross-cultural and trans-historical perspective is very difficult. However, I think it is clear generally that our views of sexuality and the biological nature of sexual orientation in our culture do not actually work when studying most cultures and times, particularly those cultures which have normalized significant same-sex sexual contact in some contexts.
 Grimes, Ronald. "Deeply into the Bone: Re-Inventing Rites of Passage"
 Warren, Elizabeth. "The Coming Collapse of the Middle Class"
 Turner, Victor. "The Ritual Process."
 Davis-Floyd, Robbie. "Birth as an American Rite of Passage, 2nd Edition"
 One can see an argument that race provides a critical difference between Brandenburg v. Ohio and Holder v. Humanitarian Law Project on the question of when advocating to or on behalf of a terrorist organization is Constitutionally protected.
 For example, the Sambia of New Guinea require that boys become men by fellating tribal elders, see note 1 above. Also Aristotle discusses the use and limits of pederasty in ancient Athens while Plutarch practically calls this, along with wife-swapping, the backbone of Spartan society in "The Life of Lycurgus". Note also that males having penetrative sex with male slaves was common in much of the ancient world. The imposition of the Levitican prohibition by Christians in Late Antiquity was largely unusual at that time.