The discussion here has reached the point of silliness, really. Which is why I am going to go out on a limb and suggest something that might sound radical, but I don't think it really is.
Gay marriage, issues surrounding sexual identity in general, are in that class of things that, going back to St. Paul's discussion over whether to eat meat dedicated to idols, has been called adiaphora, things of no consequence.
St. Paul had to deal with a whole lot of silliness in the churches with which he corresponded. Some of it was not just silly, but reflected a kind of practical blindness in the face of the call of the Gospel to be the new creation. One of the things with which he dealt was the way huge controversies in various congregations arose over matters of little consequence relative to the central concerns of the Gospel. He was patient, really and made clear that this was just not an issue about which anyone should care. You want to eat meat dedicated to idols? OK. You would prefer not to do so? That's OK, too. Just as long as the entire question didn't create confusion among those who were not as advanced spiritually as others. Don't do anything to make some folks stumble, is all.
While marriage is often discussed, in particular in the New Testament, it is used as a metaphor for the relationship between God and the Church. Over the course of two millennia, the terms have become switched, and what was originally considered an allegory became, in the Roman Church, a sacrament. While its sacramental status was dropped by the Reformation (it was not something instituted by Jesus Christ), it remains, in many ways, a sacrament-by-default. All one has to do is listen to the opening words of the marriage rite in any Protestant Church ("Marriage is an institution created by God . . . ") to understand how muddled is our thinking on this subject.
To me, however, the question over the status of marriage in general isn't settled, but rather begged, by this state of affairs. If the Christian west, or Christendom in general, were the only society in which marriage existed, I might be persuaded that are dealing with something vital to our spiritual as well as secular lives. It just isn't the case; marriage exists pretty much across most cultures in some form or other as the legal recognition of particular procreative relationships. It is, in other words, merely a way to control both breeding and the passing on of personal property. Making of this rather mundane, legally binding contract between two persons and the state something spiritual is lovely, but creates more problems than it is worth. Not least of these problems, of course, are matters of what kinds of marriages are or should be legally recognized. With changes in social attitudes toward women and sexual minorities, eventually the whole question of marriage becomes fraught with hazards.
While there can certainly be a spiritual dimension to marriage, it isn't inherent in the institution itself. Again, across most societies, marriage is little more than a contract with the state for regulating property disposition and procreation. You want to make more of it than that, why go right ahead. As things change, however, we should not be tied to two-thousand year old allegories that have become misunderstood, had the terms reversed, and used to beat down interracial marriages, keep and trade women as property, and now refuse to recognize that two persons of the same gender can and do enjoy the practical realities of a life lived together, without however, enjoying the benefits of state (let alone religious) sanction, to which might accrue the benefits and responsibilities inherent in such sanctioning.
Since there are, as a practical matter, several states and a couple Christian denominations that recognize same-sex unions, either by marriage or another name (Illinois' legalizing of same-sex unions goes in to effect soon, thank God), it seems to me churches in general would be far better off treating this entire matter as adiaphora. As there is so little Scriptural support for any position regarding same-sex marriage, and the entire matter does not at all impact the central issue of the Gospel - salvation by faith through grace - is seems to me that, rather than sit around and denounce one another as heretics, it might be better to set these matters to one side. People of good faith can disagree. As far as I'm concerned, it seems to me of little consequence whether or not any particular denomination or church wishes to recognize same-sex unions or not; if not, OK. If so, OK.
It is, in a word, adiaphora.