Conservatives should appreciate the organic nature of public discourse. Leave it to leftists to long for a controlled setting — like a classroom or a non-profit board meeting — in which some mediator decisively declares points won or lost. In the broader society, topics move forward and swing back to the beginning as new participants make discoveries of arguments that others thought had been addressed long ago.
Admittedly, it does take some effort to appreciate such a process, and with the ease of the Internet as a record keeper, it’s frustrating when obvious, already repeated counterarguments are not quickly found. For example, in Providence Journal columnist Ed Fitzpatrick’s discovery, on Sunday, of the “conservative case” for same-sex marriage, this is the entirety of his summary of the opposing opinion:

In opposing same-sex marriage, some conservatives cite the Scriptures and talk about values. But they could just as easily cite those sources in support of same-sex marriage, talking about the values of fidelity and commitment, fairness and equality, love and acceptance.

Fitzpatrick dutifully spends several paragraphs rebutting the appeal to Scripture. He subsequently offers a bait-and-switch when he addresses the appeal to procreation as the unique marker of the male-female relationship, which justifies a unique category for their intimate relationships. For that, he relies on a California judge’s questioning, of Proposition 8 supporters, what harm same-sex marriage could do to the goal of furthering stability in procreative relationships and why marriages between elderly men and women don’t have the same effect.
This is well-trodden ground. The idea of marriage is a matter of basics and obvious realities that any sentient adult can perceive: Men and women who have sex tend toward procreation. That is, they can create children with no direct intention to do so. To say that marriage is a relationship between men and women, and only men and women, is a recognition that that fact is at least sufficiently important to merit a special institution that society can leverage to maximize the number of children born and raised by the parents who created them. The age of the spouses has no effect on this simple equation, not the least because it isn’t obvious to all who see the couple at the grocery store that their relationship has never been procreative.
Fitzpatrick is correct, in his quoting of New York Times columnist David Brooks, that “we should consider it scandalous that two people could claim to love each other and not want to sanctify their love with marriage and fidelity.” The problem is that, relative numbers being what they are, it would be a travesty to allow the “conservative case” for same-sex marriage to come at the expense of the conservative case for opposite-sex marriage. Several of Fitzpatrick’s sources emphasize that marriage and fidelity are on the ropes, with adultery and the serial adultery of divorce and remarriage, but they leave it as implied (not argued) that broadening marriage’s scope will somehow strengthen its force.
This 2008 article by Joe Kort, in Psychology Today, comes to mind:

I’ve wanted to write an article on this topic ever since I began working with a gay male couple who told me that they were monogamous. After several months, however, they informed me they had had a three-way. When I asked if they had changed from monogamy, they said, “No.”
I was confused. Maybe I hadn’t gotten the correct information in our initial consultation? I told them, “I thought you told me you were monogamous,” and they said, “We are.” Now I was REALLY confused! So I said, “But you just told me you were monogamous.”
Their reply was, “We are monogamous. We only have three-ways together, and are never sexual with others apart from each other.”

Here’s something a little closer to home, by Timothy Cavanaugh and David Abbott, in Medicine and Health Rhode Island (emphasis added):

Without stereotyping gay men as promiscuous, providers need to address the role that sexual activity may play in their patients’ lives. In a recent behavioral survey of gay men, 75% had more than one partner in the past year; 27% had 10 or more. Some gay men find their sex partners at bars, bathhouses, private sex parties, public “cruising” areas like parks and rest stops, and, increasingly, on the internet. Others have traditional dating experiences, and many gay men have been happily partnered for years, despite their inability to legally marry. A longstanding relationship does not ensure sexual monogamy: many gay men have sex outside their relationships, often with the consent of their primary partners.

Gay columnist David Benkof notes that even mainstream homosexuals might not mean “monogamy” when they say “monogamy.” Similarly to the concept of marriage, they’ve redefined the word to suit their subculture.
None of this is meant to derail the debate into accusations of wickedness or to prove, in some sense, that homosexuals aren’t worthy of marriage. Indeed, the weakness of heterosexuals is what makes a strong marital culture so important. The point is that introducing a radical element to the faltering institution of marriage won’t affirm its principles, it will collapse them.
For all the talk of marriage and fidelity among them — and criticism of conservatives who’ve divorced — it’s a glaring omission that nobody who finds the “conservative case” for same-sex marriage persuasive advocates for tighter divorce laws in the mix. Is nobody concerned with the practical and legal risks of modifying a designation with far-reaching implications (such as immigration, rights to employment benefits and pensions, and even protections against testifying in court) in such a way as to include any adult pair, while allowing that designation to remain easily dissoluble?
Of course, it’s been clearly stated for years that homosexuals want marriage as it is: little more than a cultural nod with benefits. As I explained in my 2005 National Review article on Andrew Sullivan and his advocacy for same-sex marriage, they want the full range of choices available to heterosexuals, whatever those choices might be and no matter the relevance of a given choice to them.
By the nature of their relationships, homosexuals cannot create children as a nearly accidental matter. Therefore, while society should certainly develop some mechanism to encourage them toward more stable relationships, doing so should not be accomplished by flatly denying the consequence of that which makes opposite-sex intimate relationships unique.