I'm a liberal Democrat. And I do not favor same-sex marriage. Do those positions sound contradictory? To me, they fit together.

Many seem to believe that marriage is simply a private love relationship between two people. They accept this view, in part, because Americans have increasingly emphasized and come to value the intimate, emotional side of marriage, and in part because almost all opinion leaders today, from journalists to judges, strongly embrace this position. That's certainly the idea that underpinned the California Supreme Court's legalization of same-sex marriage.

But I spent a year studying the history and anthropology of marriage, and I've come to a different conclusion.

Marriage as a human institution is constantly evolving, and many of its features vary across groups and cultures. But there is one constant. In all societies, marriage shapes the rights and obligations of parenthood. Among us humans, the scholars report, marriage is not primarily a license to have sex. Nor is it primarily a license to receive benefits or social recognition. It is primarily a license to have children.

In this sense, marriage is a gift that society bestows on its next generation. Marriage (and only marriage) unites the three core dimensions of parenthood -- biological, social and legal -- into one pro-child form: the married couple. Marriage says to a child: The man and the woman whose sexual union made you will also be there to love and raise you. Marriage says to society as a whole: For every child born, there is a recognized mother and a father, accountable to the child and to each other.

These days, because of the gay marriage debate, one can be sent to bed without supper for saying such things. But until very recently, almost no one denied this core fact about marriage. Summing up the cross-cultural evidence, the anthropologist Helen Fisher in 1992 put it simply: "People wed primarily to reproduce." The philosopher and Nobel laureate Bertrand Russell, certainly no friend of conventional sexual morality, was only repeating the obvious a few decades earlier when he concluded that "it is through children alone that sexual relations become important to society, and worthy to be taken cognizance of by a legal institution."

Marriage is society's most pro-child institution. In 2002 -- just moments before it became highly unfashionable to say so -- a team of researchers from Child Trends, a nonpartisan research center, reported that "family structure clearly matters for children, and the family structure that helps children the most is a family headed by two biological parents in a low-conflict marriage."

All our scholarly instruments seem to agree: For healthy development, what a child needs more than anything else is the mother and father who together made the child, who love the child and love each other.

For these reasons, children have the right, insofar as society can make it possible, to know and to be cared for by the two parents who brought them into this world. The foundational human rights document in the world today regarding children, the 1989 U.N. Convention on the Rights of the Child, specifically guarantees children this right. The last time I checked, liberals like me were supposed to be in favor of internationally recognized human rights, particularly concerning children, who are typically society's most voiceless and vulnerable group. Or have I now said something I shouldn't?

Every child being raised by gay or lesbian couples will be denied his birthright to both parents who made him. Every single one. Moreover, losing that right will not be a consequence of something that at least most of us view as tragic, such as a marriage that didn't last, or an unexpected pregnancy where the father-to-be has no intention of sticking around. On the contrary, in the case of same-sex marriage and the children of those unions, it will be explained to everyone, including the children, that something wonderful has happened!

For me, what we are encouraged or permitted to say, or not say, to one another about what our society owes its children is crucially important in the debate over initiatives like California's Proposition 8, which would reinstate marriage's customary man-woman form. Do you think that every child deserves his mother and father, with adoption available for those children whose natural parents cannot care for them? Do you suspect that fathers and mothers are different from one another? Do you imagine that biological ties matter to children? How many parents per child is best? Do you think that "two" is a better answer than one, three, four or whatever? If you do, be careful. In making the case for same-sex marriage, more than a few grown-ups will be quite willing to question your integrity and goodwill. Children, of course, are rarely consulted.

The liberal philosopher Isaiah Berlin famously argued that, in many cases, the real conflict we face is not good versus bad but good versus good. Reducing homophobia is good. Protecting the birthright of the child is good. How should we reason together as a society when these two good things conflict?

Here is my reasoning. I reject homophobia and believe in the equal dignity of gay and lesbian love. Because I also believe with all my heart in the right of the child to the mother and father who made her, I believe that we as a society should seek to maintain and to strengthen the only human institution -- marriage -- that is specifically intended to safeguard that right and make it real for our children.

Legalized same-sex marriage almost certainly benefits those same-sex couples who choose to marry, as well as the children being raised in those homes. But changing the meaning of marriage to accommodate homosexual orientation further and perhaps definitively undermines for all of us the very thing -- the gift, the birthright -- that is marriage's most distinctive contribution to human society. That's a change that, in the final analysis, I cannot support.

David Blankenhorn is president of the New York-based Institute for American Values and the author of "The Future of Marriage."