Last year, two well-known Catholics from New York got into a public argument over the meaning of marriage. The dispute started when Mayor Rudolph Giuliani proposed that the New York City Council pass new "domestic partnership" legislation that would effectively make unmarried cohabiting couples, both heterosexual and homosexual, the legal equal of married couples in a wide range of matters, from housing to death benefits to city contracts. The Roman Catholic archbishop of New York, John Cardinal O'Connor, objected, saying: "Marriage matters supremely to every person and every institution in our society. It is imperative, in my judgment, that no law be passed contrary to natural moral law and Western tradition by virtually legislating that marriage does not matter."

In reply, the mayor begged to differ: "You know, we have a division of church and state in the United States, and it's a healthy one. We're all here because people left other places because someone wanted to enforce their religious viewpoint as the view of the state." Judging from public and political reaction, the mayor won the debate handily. The domestic-partnership proposal is now law.

But this exchange deserves a moment of attention, for Mayor Giuliani made a remarkable assertion. The goal of maintaining a privileged legal status for marriage, the mayor tells us, is essentially a sectarian religious objective. It would be similar to insisting, say, that all public-school students wear yarmulkes, or that all elected officials publicly affirm the Nicene Creed. Obviously, the potential abuse that stems from state-mandated religious practice is precisely why the Founders disestablished religion in the first place, instituting the separation of church and state. ...