Two days after legalizing same-sex marriage in New York, Gov. Andrew Cuomo marched in the New York City gay pride parade. “You’re going to see this message resonate all across the country now,” he predicted. “If New York can do it, it’s O.K. for every other place to do it.”

But most states can’t do what New York did. Their legislatures can’t legalize gay marriage, because their voters have passed ballot measures that prohibit it under their state constitutions. The ballot measures were enacted years ago, when gay marriage was unpopular. Now many of the old voters who opposed same-sex marriage are being replaced by young voters who support it. But the old electorate, through its constitutional amendments, has handcuffed the new electorate. The living are being ruled by the dead.

Twenty years ago, gay marriage was a fringe idea. Ten years ago, it was gaining support, but opponents still outnumbered supporters. In a 2003 Pew survey, taken shortly before the Massachusetts Supreme Judicial Court ruled that gay couples were entitled to marry, the national margin of opposition was 53 to 38 percent. In a 2004 Gallup survey conducted shortly after the Massachusetts ruling, it was 55 to 42 percent. In a Washington Post/ABC News survey, it was 55 to 39 percent.

Conservatives hustled to put anti-gay-marriage constitutional amendments on state ballots. One reason for their haste was the Massachusetts ruling, which set off alarms that judges might impose gay marriage in other states. Another reason was that Republicans needed a scary issue to mobilize the religious right in the 2004 elections. But the third reason was that the polls, from a conservative standpoint, were moving in the wrong direction. Conservatives often fret that traditional moral assumptions are unraveling. In the case of gay marriage, they’re right.

So they took the issue to the polls. By 2002, voters in three states had approved constitutional amendments against gay marriage. In 2004, another 13 states joined the list. Two more followed in 2005, eight more in 2006, and three more in 2008. That’s 29 states.

All of these measures passed with majority support. But the majorities that passed them were dwindling. Nevada passed its amendment in 2000 with 70 percent support. Two years later, it passed the same amendment with 67 percent support. By last year, when Nevadans were asked, “Do you support or oppose legalizing gay marriage in Nevada,” only 46 percent said they opposed it. Thirty-five percent said they supported it. Oregon passed an amendment in 2004 with 57 percent support. Now, when Oregonians are asked whether “same-sex marriage should be legal or illegal,” only 42 percent say it should be illegal. Forty-eight percent say it should be legal. In 2006, Virginia passed an amendment with 57 percent support. Now, when Virginians are asked whether “it should be legal or illegal for gay and lesbian couples to get married,” they choose legality by a plurality of 47 to 43 percent.

These shifts aren’t clear-cut, because the poll questions differ from the wording of the ballot measures. In some states, the shifts haven’t yet produced majorities for legalizing gay marriage. In other states, polls have stalled or wobbled in the opposite direction. But the trend of the past decade is unmistakable. In Gallup’s national survey, support for legalization has gone from a 55-42 deficit in 2004 to a 53-45 advantage. In the Post/ABC poll, it has gone from a 55-39 deficit to a 53-44 advantage. In Pew’s samples, which lean more conservative, it has gone from a 59-32 deficit to a 45-46 split. And in CNN/Opinion Research surveys, it has gone from a 53-44 deficit in 2008 to a 51-47 advantage.

The cultural slide toward gay marriage has been evident in polls for decades. In California’s Field surveys, when voters were asked whether they “approve or disapprove of California allowing homosexuals to marry members of their own sex and have regular marriage laws apply to them,” the disapproval rate declined from 59 percent in 1977 to 56 percent in 1997 to 50 percent in 2004. Meanwhile, the approval rate climbed from 28 percent to 38 percent to 44 percent. As of last year, the balance had switched: By 51 to 42 percent, Californians approved of gay marriage.

In New York, shifting public opinion made all the difference. Seven years ago, in a Quinnipiac University poll, New Yorkers opposed “a law that would allow same-sex couples to get married” by 55 to 37 percent. This year, in the same poll, they favored the idea by 56 to 38 percent. A similar reversal shows up in Siena Research surveys, where New Yorkers have gone from opposing gay marriage to favoring it by 58 to 36 percent. Cuomo’s private polling confirms the trend. He used these numbers to assure anxious state senators that the electorate was embracing same-sex marriage. To prove it, his allies flooded senators with postcards from constituents. Five senators switched sides, and the bill passed.

That can’t happen in California, Virginia, or the other 27 states where constitutional amendments forbid gay marriage. In those states, legislators are bound not by today’s constituents but by yesterday’s voters. Many of those voters aren’t even around anymore. The strongest support for banning gay marriage, nationally and in nearly every state that has faced a referendum, has come from old people. In Arizona, California, Oregon, and Wisconsin, voters below the age of 30 opposed ballot measures to ban gay marriage but were outvoted by their elders. In Michigan, Ohio, and Virginia, young voters split almost evenly. Today, you can see the same pattern in Ohio, Oregon, South Dakota, and other state and national polls. Gay marriage is becoming a majority position in part because people are changing their minds, and in part because a generation that’s OK with homosexuality is replacing a generation that wasn’t.

The question now is whether the new majority will get its way. To undo the constitutional amendments of the past decade, supporters of gay marriage will have to pass ballot measures in those states. In Nevada, they’ll have to do it twice. Passing ballot measures is hard. People tend to vote against them out of suspicion and fear, particularly when you’re messing with the constitution.

From a conservative standpoint, that’s how the system should work. The point of amending state constitutions while the polls were still against gay marriage was to protect the culture of the traditional family from the onslaught of normalized homosexuality.

But if the culture of the traditional family as enshrined in these constitutions is wrong—if marriage is moral and healthy regardless of sexual orientation—then the walls erected by those ballot measures are a prison inflicted by the old on the young. And that legacy, unlike marriage, is a bond that death alone can’t break.

(Readings I recommend: Jeffrey Goldberg celebrates the gay marriage movement as a vindication of the attraction of monogamy. David Frum argues that conservative fears about gay marriage have been falsified: “American family stability … has deteriorated much more slowly than it did in the 1970s and 1980s before same-sex marriage was ever seriously thought of.” Rudy Giuliani says, “I still favor marriage as being defined between a man and a woman … But I was very glad to see people relieved of this burden of discrimination.” Andrew Sullivan mavels that conservatives who exalted legislation over judicial activism now call New York’s law a “ tyranny of the majority.” Steve Martin wants polygamy next: “I’m gettin’ married in the mornin’! Wait. I am already married. NEW LAW REQUIRED.” A Twitter wag replies: “ Become a Mormon!”)