Ed Whalen gives it the old college try.  I've taken out his cites.  You can (and should) read the whole thing here:

As I’ve outlined, Judge Walker somehow failed to identify the opposite-sex character of marriage as one of the core characteristics of marriage throughout American history.  In their stay motion to the Ninth Circuit, Prop 8 proponents restate some of the record evidence and other authority that they presented to Walker—and that he simply ignored and claimed didn’t exist.  The rest of this post…  is excerpted from the stay motion.  “DIX” references are to defendants’ trial exhibits.  (I’ve deleted some citations and changed the “all caps” punctuation in others.)

In the words of highly respected anthropologist Claude Levi-Strauss, “the family—based on a union, more or less durable, but socially approved, of two individuals of opposite sexes who establish a household and bear and raise children—appears to be a practically universal phenomenon, present in every type of society.” The View from Afar 40-41 (1985) (Trial Exhibit DIX63); see also G. Robina Quale, A History of Marriage Systems 2 (1988) (DIX79) (“Marriage, as the socially recognized linking of a specific man to a specific woman and her offspring can be found in all societies.”).

The opposite-sex character of marriage has always been understood to be a central and defining feature of this institution, as uniformly reflected in dictionaries throughout the ages. Samuel Johnson, for example, defined marriage as the “act of uniting a man and woman for life.” A Dictionary of the English Language (1755). Subsequent dictionaries have consistently defined marriage in the same way, including the first edition of Noah Webster’s, and prominent dictionaries from the time of the framing and ratification of the Fourteenth Amendment.  [citations omitted]

Nor can this understanding plausibly be dismissed, as the court below did, as nothing more than an “artifact of a time when the genders were seen as having distinct roles in society and in marriage.” Rather, it reflects the undeniable biological reality that opposite-sex unions—and only such unions—can produce children. Marriage, thus, is “a social institution with a biological foundation.” Levi-Strauss, “Introduction,” in Andre Burguiere, et al. (eds.), 1 A History of the Family: Distant Worlds, Ancient Worlds 5 (1996). Indeed, an overriding purpose of marriage in every society is, and has always been, to approve and regulate sexual relationships between men and women so that the unique procreative capacity of such relationships benefits rather than harms society. In particular, through the institution of marriage, societies have sought to increase the likelihood that children will be born and raised in stable and enduring family units by the mothers and fathers who brought them into this world.

I emphasize those final sentences because that's where he runs into problems.

Look, there's no question that society has traditionally, for centuries, viewed marriages as between a man and woman, and the opposite-sexed-ness has always been a core component of marriage.  Point conceded.

But guess what happens when gays are allowed to marry?  People will still view marriages as taking place between a man and a woman.  The only difference is that, in the back of our minds, we will also know that same-sex couples can also get married.  An opposite-sex marriage will always be a "traditional marriage".  So what's the problem?

And the argument that "the purpose of marriage in every society… is to approve and regulate sexual relationships between men and women so that the unique procreative capacity of such relationships benefits rather than harms society"?  I'll even concede that for the purposes of argument (although in truth, society approves and honor hetero marriages without regard to whether the couples intend to procreate).

But concede as I do, guess what happens to that argument when gays are allowed to marry?  Society will still approve of marriages between men and women.  That's right, kids — even after same-sex marriage becomes legalized, opposite-sex marriage will still have a unique procreative capacity.

You see, here's the thing these conservatives don't get: The institution of marriage is not a zero-sum game.  We can recognize and honor same-sex marriages without changing even slightly the recognition and honor we give to hetero marriages and their "unique procreative capacity". Permitting gay marriage does not diminish hetero marriages, just as permitting interracial or interfaith marriages didn't diminish marriages of those of the same race or religion.

We all end up better off when we allow everyone to form stable, lifelong marriages that are honored and cherished.  

So you can make the argument that marriage among opposite-sex people is important and valued, and point to all kinds of historical data to back up that proposition.  But even that doesn't provide rationale for banning same-sex marriage.  Why is that so hard to understand?

Fortunately, people are starting to get it.

I mean, even Glenn Beck is starting to get it:

O'REILLY: Do you believe — do you believe that gay marriage is a threat to the country in any way?

BECK: A threat to the country?

O'REILLY: Yeah, it going to harm the country?

BECK: No, I don't. Will the gays come and get us?

O'REILLY: OK. Is it going to harm the country in any way?

BECK: I believe — I believe what Thomas Jefferson said. If it neither breaks my leg nor picks my pocket, what difference is it to me?

O'REILLY: OK, so you don't. That's interesting. 

UPDATE:  I see Ross Douthat has made the same error.  His argument is a bit weedy, even in context, but here it is:

The interplay of fertility, reproductive impulses and gender differences in heterosexual relationships is, for want of a better word, “thick.” All straight relationships are intimately affected by this interplay in ways that gay relationships are not. (And I do mean all straight relationships. Because they’ve grown up and fallen in love as heterosexuals, the infertile straight couple will experience their inability to have children very differently than a same-sex couple does. Similarly, even two eighty-nine-year-old straights, falling in love in the nursing home, will be following relational patterns — and carrying baggage, no doubt, after eighty-nine years of heterosexual life! — laid down by the male-female reproductive difference.) This interplay’s existence is what makes it possible to generalize about the particular challenges of heterosexual relationships, and their particular promise as well. And the fact that this interplay determines how and when and whether the vast majority of new human beings come into the world is what makes it possible to argue — not necessarily convincingly, but at least plausibly! — that both state and society have a stronger interest in the mating rituals of heterosexuals than in those of gays and lesbians.

Douthat is arguing that there is, and always has been within society, a strong interplay between fertility and reproductive impulses, and that interplay exists in heterosexual relationships — even before we heterosexuals get married.  Okay, fine.  And where does he go off the rails?  At the end…. "both state and society have a stronger interest in the mating rituals of heterosexuals than in those of gays and lesbians".

And we're back to the zero-sum game.  Even if society has a stronger interest in the mating rituals of homosexuals (even barren ones, as Douthat argues), so what?  Must we pick only one thing we are interested in as a society?  If I have an interest in the New York Giants, does that mean I can't have an interest in the Boston Red Sox, particularly when they're not even playing the same game?