The country used to be unanimous in rejecting gay marriage. But that consensus, like the polar ice sheets, is showing some cracks. Vermont recently became the fourth state to allow gays to wed, and New York will probably be next. Elsewhere, marriage remains as Miss California prefers -- solely between a man and a woman.

It's at moments like this that the framers of the Constitution begin to look even wiser than usual. Somehow they anticipated that people in Massachusetts would not want to live under exactly the same laws as people in Mississippi. So they set up a system known as federalism, which allows different states to choose different policies. Thus we simultaneously uphold majority rule and minority rights.

This, at least, is how federalism is supposed to operate -- letting subsets of the national population get their way in their own locales. There's only one hitch: In this case, it doesn't quite work that way.

Why not? Because of a huge imbalance created by that longtime nemesis of state sovereignty -- the federal government. Under the 1996 Defense of Marriage Act (DOMA), Virginia has complete authority to deny the privileges and responsibilities of marriage to same-sex partners. But Iowa doesn't have the complete authority to grant them.

Oh, Iowa can provide recognition to gay marriages under all its laws and policies. But that's a surprisingly small part of what marriage encompasses. Under federal law, there are more than 1,100 rights and privileges that go with being a husband or wife. And none of them is available to married same-sex couples.

Under federal law, a person may transfer property to a spouse tax-free. Married couples may file their income taxes jointly. Someone whose spouse dies is assured Social Security survivor's benefits. A married person has the authority to make medical decisions for an incapacitated partner.

Or say you're an American citizen living in this country who marries a foreigner. Normally, you would be entitled to bring your beloved to this country to live permanently and become a citizen.

But if you're both of the same sex, you can forget all of the above. Even though Iowa might like to put heterosexual and homosexual married couples on the same footing, it can't, because the federal statute blocks the way.

"In determining the meaning of any Act of Congress, or of any ruling, regulation, or interpretation of the various administrative bureaus and agencies of the United States," says DOMA, "the word 'marriage' means only a legal union between one man and one woman as husband and wife."

That decree may sound reasonable: Since most Americans and most states reject same-sex marriage, federal policy should as well. But it conflicts with how the nation has handled marriage up till now, which is to leave it up to individual states to decide who may wed -- and then honor those diverse choices.

Some states, for instance, allow marriages between first cousins; others forbid it. Some states allow 15-year-olds to marry with parental consent, while most set the minimum age higher.

And the feds? They have consistently observed a policy of staying the hell out. Washington doesn't tell Colorado and New York which marriages it will acknowledge. Colorado and New York tell it.

Not so with same-sex unions. Under DOMA, the federal government insists that some marriages are not marriages.

That's particularly hard to justify because the other major provision of the law bends over backward to protect state authority over matters marital. It says no state is obligated to recognize a same-sex marriage that took place somewhere else. Gays married in Vermont magically become single when they venture into New Hampshire.

This part of the law goes beyond the norm to accommodate different preferences. Usually, states are obligated to enforce contracts made in other states. Back in the segregationist years, Southern states often honored interracial marriages transacted beyond their borders even though they regarded them as "so unnatural that God and nature seem to forbid them."

Given the strong feelings about gay marriage, the local option is the best option. States that abhor the idea should be free to implement policies reflecting that sentiment. But the other side should have exactly the same prerogative: giving both heterosexual and homosexual couples access to marriage in full.

Our system, unlike Mao's China, is supposed to let a hundred flowers bloom. But for the best growth, the federal sun has to shine on all of them.