Editor's note: this piece is co-authored by Sherif Girgis and Robert P. George.

The attractive civil rights rhetoric of "marriage equality" masks a profound error about what marriage is.

Of course, if marriage were simply about recognizing bonds of affection or romance, then two men or two women could form a marriage just as a man and woman can. But so could three or more in the increasingly common phenomenon of group ("polyamorous") partnerships. In that case, to recognize opposite-sex unions but not same-sex or polyamorous ones would be unfair -- a denial of equality.

But marriage is far more than your emotional bond with "your Number One person," to quote same-sex marriage proponent John Corvino. Just as the act that makes marital love also makes new life, so marriage itself is a multilevel -- bodily as well as emotional -- union that would be fulfilled by procreation and family life. That is what justifies its distinctive norms -- monogamy, exclusivity, permanence -- and the concept of marital consummation by conjugal intercourse.

It is also what explains and justifies the government's involvement in marriage.

The government takes no notice of companionship for its own sake, romantic or otherwise. But it has powerful reasons to ensure that whenever possible, children have the benefit of being reared by the mom and dad whose union gave them life.

All human beings are equal in dignity and should be equal before the law. But equality only forbids arbitrary distinctions. And there is nothing arbitrary about maximizing the chances that children will know the love of their biological parents in a committed and exclusive bond. A strong marriage culture serves children, families and society by encouraging the ideal of giving kids both a mom and a dad.

Indeed, if that is not the public purpose of marriage law, then the "injustice" and "bigotry" charges comes back to bite most same-sex marriage supporters.

If marriage is just the emotional bond "that matters most" to you -- in the revealing words of the circuit judge who struck down California Proposition 8 -- then personal tastes or a couple's subjective preferences aside, there is no reason of principle for marriage to be pledged to permanence. Or sexually exclusive rather than "open." Or limited to two spouses. Or oriented to family life and shaped by its demands.

In that case, every argument for recognizing two men's bond as marital --equality, destigmatization, extending economic benefits -- would also apply to recognizing romantic triads ("throuples," as they are now known). Refusing such recognition would be unfair -- a violation of equality -- if commitment based on emotional companionship is what makes a marriage.

But don't take our word for it. Many prominent leaders of the campaign to redefine marriage make precisely the same point. (We provide many more examples, and full citations, in the amicus brief we filed with the Supreme Court on the harms of redefining marriage.)

University of Calgary Professor Elizabeth Brake supports "minimal marriage," in which people distribute whichever duties they choose, among however many partners, of whatever sex.

NYU Professor Judith Stacey hopes that redefining marriage would give marriage "varied, creative, and adaptive contours ..." and lead to acceptance of "small group marriages." In the manifesto "Beyond Same-Sex Marriage," 300 leading "LGBT and allied" scholars and activists call for the recognition of multiple partner relationships.

Influential columnist and "It Gets Better" founder Dan Savage encourages spouses to adopt "a more flexible attitude" about sex outside their marriage. Journalist Victoria Brownworth cheerfully predicts that same-sex marriage will "weaken the institution of marriage."

"It most certainly will do so," she says, "and that will make marriage a far better concept than it previously has been."

Author Michelangelo Signorile urges same-sex partners to "demand the right to marry not as a way of adhering to society's moral codes but rather to debunk a myth and radically alter an archaic institution." They should "fight for same-sex marriage and its benefits and then, once granted, redefine the institution of marriage completely, because the most subversive action lesbians and gay men can undertake ... is to transform the notion of 'family' entirely."

These leading same-sex marriage advocates are correct.

Redefining marriage would, by further eroding its central norms, weaken an institution that has already been battered by widespread divorce, out-of-wedlock child bearing and the like.

Listen: Voices from the Southern closet

People who think that would be good for children, families and society generally should support "marriage equality." People who believe otherwise shouldn't be taken in by the deceptive rhetoric.