His book, "The Future of Marriage," published last month by Encounter Books, explains why he doesn't believe same-sex marriage will serve that cause. But given the charged nature of the subject, his book may also set a record for optimism about the human capacity for rational discussion.

Mr. Blankenhorn, who opposes same-sex marriage, believes that the national debate about the issue can be rescued from the polarized clash of gut reactions, religious injunctions, emotional appeals and accusations of bigotry. He even believes the debate could provide "an invaluable opportunity for Americans to have a serious national discussion about marriage's meaning and future."

The problem with that debate until now, as he sees it, is that "almost always, the main focus is 'gay,' not 'marriage.' "

Mr. Blankenhorn cites what he calls the "wafer-thin" definitions of marriage that increasingly turn up in court decisions and polemical articles about same-sex ties: "a unique expression of a private bond and profound love"; "a private arrangement between parties committed to love"; "the exclusive commitment of two individuals to each other."

Some of this commitment talk sounds sweet, and some of it, like "committed, interdependent partnerships between consenting adults," sounds more like a real estate transaction than a marriage. But for Mr. Blankenhorn, these definitions miss the point. He is amused, for instance, at their neo-Victorian avoidance of any mention of sex. Similarly, these definitions dodge any mention of children and parenthood. They emphasize marriage as private and too diverse ("unique") to be pinned down.

On the contrary, Mr. Blankenhorn writes, marriage is a "social institution," a set of shared understandings and public meanings that shape expectations and conduct. Marriage has evolved and, yes, may be "constantly evolving"; here Mr. Blankenhorn moves through biology, prehistory, history and anthropology, from ancient Mesopotamia to the Trobriand Islands. But marriage fundamentally involves sexual intercourse and the affiliation - emotionally, practically and legally - between any child created and both parents.

"If this book had a subtitle," Mr. Blankenhorn writes, "it would be 'An Argument About Institutions.' " He captures his ideas of marriage as an institution with a quotation from a wedding sermon that the German theologian Dietrich Bonhoeffer sent to a young couple from his Nazi prison cell. Bonhoeffer, soon to be executed for his role in a plot against Hitler, wrote, "It is not your love that sustains the marriage, but from now on, the marriage that sustains your love."

Mr. Blankenhorn readily admits that the "deinstitutionalization" of marriage that he fears - the redefinition of what he considers the nation's "most pro-child institution" as a private adult relationship stripped of public meaning - has been under way for a long time. Deeply rooted in American individualism and the quest for self-fulfillment, that redefinition "has been growing for decades, propagated overwhelmingly by heterosexuals." Same-sex marriage only further erodes marriage as a pro-child institution, he believes.

Mr. Blankenhorn wishes it weren't so. Unlike many other opponents of same-sex marriage, he explicitly recognizes the rights and needs of gay men and lesbians to be respected and accepted and to form "loving, stable partnerships."

The debate is not "a simple issue of good versus bad," he writes. "The real conflict is between one good and another: the equal dignity of all persons and the worth of homosexual love, versus the flourishing of children. On each side, the threat to something important is real. It wastes everyone's time to pretend that this question is an easy one, and that only bad people can fail to see the right answer."

Is this conflict really as inescapable as Mr. Blankenhorn believes? Jonathan Rauch, for one, doubts it, as the title of his book, "Gay Marriage: Why It is Good for Gays, Good for Straights and Good for America," suggests. In his book, published by Times Books in 2004, Mr. Rauch argues that legalizing same-sex marriage will actually "shore up marriage's unique but eroding status."

"How I wish he were right!" Mr. Blankenhorn replies. He contrasts Mr. Rauch's views with those of numerous social scientists and legal theorists who have long been critics of marriage and now suddenly support same-sex marriage precisely because they believe it will destabilize and "deconstruct" what they consider an oppressive institution.

At this point, one wonders whether academic ideology and legal theory aren't upstaging the lived experiences of gay and lesbian couples. But as long as the movement for same-sex marriage takes place largely in the courts rather than legislatures (New York could prove to be a significant exception), academic ideology and legal theory will inevitably weigh heavily.

Meanwhile, Mr. Blankenhorn hopes his book will stimulate "a better conversation" between opponents and proponents of same-sex marriage. In a phone interview Thursday, he said his own recognition of "many good reasons to support gay marriage" had been matched by some proponents' recognition of "legitimate reasons to be concerned" and that, therefore, "being opposed to gay marriage is not necessarily the expression of bigotry."

"Anything that causes an interesting new conversation where both sides recognize the validity of some of the other side's concerns," he said, "who knows, maybe an interesting new dynamic could emerge."

In fact, "The Future of Marriage" may have much to say to many others who, like Mr. Blankenhorn, oppose same-sex marriage. It could be profitably read, for example, by the many conservative allies, especially religious ones, that Mr. Blankenhorn has earned through his work on behalf of strengthening the family.

His book does not explicitly address these allies, some of whom have been as adamant in declaring support for same-sex marriage morally unthinkable as supporters have been adamant in treating opposition as reactionary. "I didn't want to start finger-pointing and set myself up as an arbiter of who is a bigot," Mr. Blankenhorn said. "I wanted to model a different way of arguing."

That way of arguing concludes with his own list of proposals, including government assistance, not for fighting same-sex marriage but simply for strengthening marriage as an institution.

"To the degree that it makes any sense to oppose gay marriage," he writes, "it makes sense only if one also opposes with equal clarity and intensity the other main trends pushing our society toward post-institutional marriage."

END