I missed an excellent post by Jane Galt on why she doesn't have a position for or against legal recognition of same-sex when it first came out two years ago. I don't know whether her stance has changed since then, but her point is still good:
The argument that gay marriage will not change the institution of marriage because you can't imagine it changing your personal reaction is pretty arrogant. It imagines, first of all, that your behavior is a guide for the behavior of everyone else in society, when in fact, as you may have noticed, all sorts of different people react to all sorts of different things in all sorts of different ways, which is why we have to have elections and stuff. And second, the unwavering belief that the only reason that marriage, always and everywhere, is a male-female institution (I exclude rare ritual behaviors), is just some sort of bizarre historical coincidence, and that you know better, needs examining. If you think you know why marriage is male-female, and why that's either outdated because of all the ways in which reproduction has lately changed, or was a bad reason to start with, then you are in a good place to advocate reform. If you think that marriage is just that way because our ancestors were all a bunch of repressed bastards with dark Freudian complexes that made them homophobic bigots, I'm a little leery of letting you muck around with it.
Certainly I don't believe that my behavior is a guide for that of any other person in society. Indeed, I hope that it isn't such a guide in all respects, as there are many things I don't do (coach children's sports, take in abandoned pets, etc.) that I am very glad to see other people doing.
However, I am wary of thinking that another human being's life is so alien to mine that he must have thought processes that bear no resemblance to my own. Therefore, Jane's argument that a heterosexual high school dropout in Tuscaloosa may be turned off getting married because he knows that homosexuals are getting married -- without any explanation of what the thought process of that Tuscaloosan might be -- isn't very convincing.
In Jane's three historical examples (the increase in income tax, theextension of benefits to unwed mothers and the loosening of divorce law), I could have found someone before each change who would say she would be the person the reformers assumed didn't exist. Socialists in America favored higher tax rates than 10% at the time of the 16th Amendment's writing; 1950s women who'd married worthless husbands just because they had gotten knocked up would have gladly traded those husbands for welfare benefits; 19th century couples who couldn't stand each other but were not egregiously adultering nor severely beating on one another would have said they'd divorce immediately upon the loosening of the laws. That the reformers were unable to imagine the existence of people other than themselves does not meant that such people didn't exist.
If the opponents of same-sex marriage can find me someone who is willing to get married with the marriage law as it is, but definitely won't be getting married if the law changes, I'd take seriously the argument that same-sex marriage will devalue marriage for heterosexuals. As it is, the only way I can imagine holding that position is if my abhorrence of homosexuality is so great that I refuse to be part of anything that homosexuals are (for example, if I were the kind of person who would quit my job if my employer hired homosexuals). Quite possibly such people exist, but I'm unwilling to assume their existence, especially in sufficiently great numbers that it is more important to keep them getting married than to allow same-sex couples to marry.
The social change created by state approval of same-sex marriage seems likely to run the other way, particularly when one looks at Jane's examples: if the state approves X, society is more likely to think X isn't so bad. If a direct tax on non-labor income is approved, Americans' resistance to the notion of such a tax will decrease. If the law grants the support of welfare benefits to unwed mothers, unwed mothers will be more acceptable. If the law permits people to divorce for reasons short of daily battery, divorcing on lesser grounds will garner less criticism.
Therefore, I completely understand why those who do not want homosexuality to become socially acceptable oppose same-sex marriage. Even if they feel sympathy for homosexual individuals and the problems caused by the inability to wed, they think those problems are insufficient to balance out the great evil of accepting homosexuality. For those of us who don't perceive homosexuality qua homosexuality (that is, merely the preference for one sex instead of another, without regard for whatever "lifestyle" is associated with homosexuality) to be so inferior to heterosexuality that it must be discouraged, however, this is not a good argument. I would be delighted if the legalization of same-sex marriage led to greater acceptance of homosexuality, and I think it may have that long-run effect as the Tuscaloosa teenager sees that his gay neighbors are going to work, raising a family and otherwise behaving much like everyone else. Thus they are not to be abhorred, and an institution that includes them need not be avoided.
I don't think that marriage was originated by people who were homophobic bigots; I think that marriage arose in order to ensure that men who impregnated women were responsible for the offspring that resulted. Because in those dark days before baby's daddy testing, men couldn't be sure if a woman's offspring were genetically their own, women began having to promise that they were only having sex with the man they wanted to help them provide for the children. Because a woman who'd given a man a monopoly on her sexuality had done so on the assumption that he would be a good provider and was unhappy when he got another woman knocked up and started dividing his resources between two families, men began having to promise that they were only having sex with the woman whose sexuality they wanted to monopolize. Where there was either a shortage of men, or some of the men had such a surplus of resources that they could provide for multiple families, polygamy was more likely to arise. (I'd consider the practice of many wealthy European men to maintain both a wife and her family, as well as concubines and their children, to be functionally polygamous; the concubine still was expected to be faithful to the provider.)
Because same-sex couples could not impregnate one another, there was no need for law and society to require them to be faithful to and provide for one another. If all the lesbians or gay men in a society were promiscuous, no great harm resulted to the society, but no great benefit redounded from their fidelity, so there was no reason to put any social pressure on them to be sexually faithful or to enter an institution that would require their sexual fidelity. Therefore this argument -- essentially, that straights get pregnant accidentally and thus need marriage to push them together, but gays don't -- is a reasonable one historically.
However, those historical conditions no longer wholly prevail. Men now can know whether offspring are theirs without having had to keep a woman from leaving the harem. Women now are more capable of bringing in the same level of resources as men, so having two women co-parent can provide sufficient resources for raising children. The legal content of marriage has changed. Instead of being necessary to ensure that men weren't wasting their resources on another man's children, and that women weren't wasting their fidelity on a man who was splitting his resources, the law now puts more weight on the couple's non-child-related duties toward one another. Men and women are treated equally in these duties; there is no longer a special duty of support that applies to men but not women, nor a special duty of fidelity that applies more strictly to women than to men.
As parentage has become readily discoverable despite promiscuity, and gender roles have become less important, marriage in American law now focuses on a couple's commitment for mutual support, and to provide as much support for any resulting children as they would have to do if they were not married. (I don't know of a separate neglect crime for married parents that is different for the neglect law applied to nonmarried parents; a custodial parent has an obligation to ensure the child's well-being, and a non-custodial parent must provide financial support.)
And of course, our society is much more complex and sophisticated. When there were merely inept healers rather than hospitals, the legally-enforceable ability to visit a spouse in the hospital or to make end-of-life decisions wasn't important; when there was no insurance, the ability to share coverage wasn't important. Almost all of the multitudinous reasons why same-sex couples want to marry were irrelevant when the institution of marriage first arose.
So the reasons that once existed to apply marriage only to male-female couples have diminished tremendously, while the reasons to allow same-sex couples to marry as well have increased enormously. Again, unless one sees homosexuality as a sufficiently great evil that it needs to remain socially unacceptable, or there are people who abhor homosexuality so much that they would abstain from marriage in order to avoid joining the same legal institution that accepted gay people and such people are so numerous or likely to influence others into the same mindset that their abstention will outweigh the number of homosexuals who will benefit, I see no reason to deny legal recognition to same-sex marriage in 21st century America.