Like it or not, this question can no longer be avoided: What is marriage?

For thousands of years, societies have held that marriage is a relationship between a man and a woman, and their laws have reflected this. But the Massachusetts Supreme Judicial Court disagrees. It says that marriage is "an evolving paradigm" and that same-sex couples have a fundamental right to marry. Which is it? How we decide this question--and it will be decided, one way or the other--will shape the future of our society and the course of constitutional government in the United States.

This question has been sharply focused by calls for a constitutional amendment upholding marriage as a union between one man and one woman--calls recently endorsed by President Bush.

And don't think this issue is going to go away. Judges have forced the issue upon the nation, beginning with a trial court judge in Hawaii, then a Superior Court judge in Alaska, then the Vermont Supreme Court and now the Massachusetts Supreme Judicial Court.

In recent weeks, San Francisco officials have issued thousands of marriage licenses to same-sex couples, contrary to state law, and sued to have a judge declare California's marriage laws discriminatory and unconstitutional in that state.

A city clerk in New Mexico issued several same-sex marriage licenses before the attorney general of that state intervened to halt this interpretation of state laws. The mayor of New Paltz, N.Y., noting ambiguity in the laws of his state, has performed several same-sex marriages. Chicago Mayor Richard Daley has voiced his support for same-sex marriage. In Oregon, Multnomah County (which includes Portland) is issuing same-sex marriage licenses but stopping short of performing marriage ceremonies.

The effect of the judicial decisions and the intent of the litigation strategy behind them--not to mention the actions of local officials, who intend to force additional judicial decisions--is to redefine marriage in accord with a new understanding of rights.

The claim is rather simple: Homosexuals, like heterosexuals, have the right to "seek autonomy" in their private relationships, including "personal decisions relating to marriage." That's what the U.S. Supreme Court said last year in Lawrence vs. Texas. The choice of marriage partner is a private matter, stemming from personal autonomy, and thus a civil right.

By extension, says the Massachusetts Court, the traditional definition of marriage is arbitrary and irrational. Not only does it violate the dignity and equality of all individuals, but banning persons of the same sex from marrying is discriminatory and serves only to reinforce prejudice against homosexuals.

It is precisely the role of the courts to override legislatures, and majority opinion, to protect these rights. It is the obligation of the federal government, and the purpose of our constitutional order, to secure these rights nationwide.

But here is the problem: Contrary to the opinion of the Massachusetts Supreme Judicial Court, marriage in not an "evolving paradigm." Nor is it an artificial, socially constructed concept.

Marriage is the formal recognition by society and the laws of society of the most profound relationship that can exist between a man and a woman, apparent and definable according to the most basic and evident truths of human nature. By nurturing children, providing them with mothers and fathers, and building and maintaining relationships between mothers, fathers and children, marriage stands as the cornerstone of the family. By virtue of its function and purpose in society, marriage is a fundamental institution necessary for societal existence and well-being.

Society has a clear and compelling interest in strengthening marriage as a social institution and sustaining marriage as the ideal place to have and raise children. As such, far from being arbitrary, irrational or capricious, it is reasonable and sensible to prefer marriage as the legal union of one man and one woman.

Indeed, if marriage is in the best interest and good of our children, our families and our society, we have an obligation to our children, our families and our society, to defend and protect the institution of marriage.

By forcing the issue of same-sex marriage, judges disregard thousands of years of custom and experience, flout the laws of every society and thumb their noses at the beliefs of every major religious tradition. And by circumventing the legislative process and overriding long-standing majority opinion to redefine marriage, and then excluding the people from so fundamental a decision as marriage, these judges--and the local officials who actively circumvent the law--undermine democracy and the rule of law.

What can be done?

A clear and compelling case must be made for the nature, substance and societal importance of marriage according to the best evidence and studies provided by social science. A strong case also must be made that redefining the institution of marriage undermines the institution, significantly weakens the case for promoting an ideal of marriage, and threatens religious liberty and private institutions.

States should review their laws concerning marriage and clarify and strengthen public-policy preferences that favor traditional marriage. To date, 39 states have defense of marriage acts.

The legislative language in those laws, however, varies widely. Efforts should be made to establish such laws in every state and clarify and strengthen existing ones.

But should we amend the U.S. Constitution?

As a supporter of limited constitutionalism, I am reluctant to change our most fundamental law.

The Constitution should be amended rarely and only for the most important of reasons. I understand and underscore the fact that our constitutional system rightly leaves the power to regulate marriage policy, like so many other things, with the states. Marriage should not become a regulatory policy matter for the federal government.

But this is no mere policy disagreement or matter for state-by-state social experimentation.

Society has never before been confronted with such a concerted legal and political effort to forcefully redefine and undermine one of its most basic institutions.

Americans are a tolerant and non-judgmental people, but they also have principles and convictions. It is wrong to accuse those who are concerned about the institution of marriage of being bigoted and discriminatory toward homosexuals.

Tolerance goes too far when it demands redefining our fundamental institutions and rejecting our deepest convictions. Yet such a redefinition and rejection is exactly what the courts are trying to thrust upon us.

Despite our reluctance--despite the significance of the endeavor and awesome task of changing the Constitution--prudence dictates that we amend the Constitution to protect the institution of marriage. The threat to marriage is unambiguous and increasingly imminent. The overriding importance of marriage and the difficulty of amending the Constitution make it crucial that we act now.

-Matthew Spalding is director of the B. Kenneth Simon Center for American Studies at The Heritage Foundation.