The conservative position on gay marriage is difficult is because it would deny to homosexual couples a public recognition of the equal stature of their union to an analogous union between mixed sexes, thus making them unhappy. Specifically, they would understandably feel marginalized, segregated, invisible, and rejected. Also, there is the implied denial of intangible (and sometimes tangible) goods connected with the status of marriage: legitimation, a sense of permanence and officiality, stability, community support, and a foundation upon which to build a legacy via [in this case adopted] children.
These are genuine goods, and the public order I would vote for would deny them to gay couples. I do not want that. I do not want gay couples to be unhappy and marginalized. To me it is a biproduct of the situation we find ourselves in--a situation in which losses will be suffered no matter what the outcome.
Conservatives suffer a disadvantage having to articulate what is, admittedly, an abstract grievance in a culture dominated by the demand for self-determination.
I will try to do so, and I may fail to persuade, but I hope this is illuminating nevertheless.
The social contract owes its existence not exclusively or even primarily to the combined wills of atomistic individuals. The social contract's allegiance is, above all, to inalienable human dignity (the source of human rights)--a dignity not given, defined, or created by the government. This dignity is recognized as already present and inherent in humans as humans, as long as humans have existed, and so it is revered--yet it is also vulnerable, and thus needs guarding. It is key to our historic understanding of government that neither kings nor majority rule are the last word; all government is subservient to human dignity, rooted in human nature, and denying nothing to anyone what belongs to each in accord with his or her humanness. They key is, what is "humanness"?
Today we conceive of "human dignity" and "human rights" in strictly individualistic terms; but if our notion of humanity is so atomistic, it is simply flat wrong. There is no humanness--there are no humans--apart from the defining structures out of which our childhoods arose. And it is not a stretch to say that the only such structure that enjoys a perennial default status (for no reason of our own machination) is the biological family. It is one thing to say that someone's dignity maintains even when, in particular cases, one arises from alternative situations ('families' in an analogous sense). It is another to deny the brute ever-presence, across time and the globe, of a reality which is so powerfully constitutive of collective human existence. To delete from public recognition, not only the brute "biology" of our individual origins, but also the consequent bonds which ideally become a child's welcome committee to the world, is to publicly, officially declare a new, different conception of human nature.
"Family" is now a household of voluntarily cohabiting individuals. Blood has no role, privilege, or status. The complete severing of the law from reality in this case represents, to me, a troubling precedent.
The pre-existing reality of families (biologically begotten) is prior to the state both in chronology and in inviolability. The state exists because of that entity, not the other way around; the state did not "invent" or "define" the fact that human beings cluster into mutually fostering bonds of spouse and spawn. It was created by them for their sake. The state does not serve citizens as atomistic individuals in every aspect of life; it serves families--both the biological and the analogous kind. But the point is that even those "analogous families" are analogous to something, which is not itself an analogy, but the real thing: blood family. It is the mold and the model for all such households. Watch me repeat myself here: it was not given, defined, or created by the government. It is recognized as already present and inherent in humans as humans, as long as humans have existed, and so it is revered--yet it is also vulnerable, and thus guarded. But in this case, it has only become vulnerable in the last thirty years.
For social conservatives there is a close connection between the public privilege granted to heterosexual marriage (the biological linchpin of family) and to the same human dignity that is the source of inalienable rights. Both are rooted in the official, state-enshrined understanding of what "humanness" is. If you can change what that word means, you can make radical alterations to the way government understand, and treats, people. It's like changing the multiplier on a computer's processor: tiny little modification, big consequences (and like an overclocked computer, the negative consequences may not be evident for some time).
The conservative position, when not blatantly bigoted or blindly religious, ultimately demands that government remember its subordination to realities that pre-existed it and have not, in their essence, changed--nor will they. This is not a debate about whether Bob and Fred can live happily ever after (they can). This is a debate about the foundation and rights of government. Democrats wish to place government over family as its creator and author (so that it can "re-author" this reality). Republicans demand that government know its place--that it not attempt to use its powers to alter that which it was built to serve.
One final, final point. There is something lost in the translation when conservatives wield signs saying "Protect marriage." People imagine that they are saying, "Protect marriage from gay people" (Hence Ron Zimmerman's great song about those who are trying to "protect marriage from people who want to get married." No. The "Protect marriage" slogan isn't about protecting marriage from gay people. It's about protecting marriage from the government--and as is most often the case, the judiciary branch.