NEW YORK (Reuters) - When New York became the sixth and by far the largest state to legalize same-sex marriage, following a grueling overtime session in the state Legislature Friday, it immediately transformed the national debate over the issue, legal experts said.

With a population over 19 million -- more than the combined population of the five states that currently allow gay marriage, plus the District of Columbia, where it is also legal -- New York is poised to provide the most complete picture yet of the legal, social and economic consequences of gay marriage.

"I think that having same-sex marriage in New York will have tremendous moral and political force for the rest of the country -- in

part because New York is a large state, and in part because it hasn't come easily,'' said Suzanne Goldberg, a professor at Columbia Law School.

The New York Assembly passed same-sex marriage legislation twice before, in 2007 and 2009, but in both cases it stalled in the state Senate, as it nearly did again this week. The bill passed late Friday after legislators agreed on language allowing religious organizations to refuse to perform services or lend space for same-sex weddings.

The new law's impact can be measured in part by the numbers at play: New York is home to more than 42,000 same-sex couples, according to an analysis of U.S. census data conducted by the Williams Institute. This means, among other things, that the number of same-sex couples living in states allowing same-sex marriage has more than doubled overnight.

REAL-WORLD DATA

If a significant portion of those couples choose to marry, it could provide a wealth of new information about the practical economic effects of such legislation, from employment and retirement benefits to divorce rates and wedding and tourism industries, said New York University Law School professor Arthur Leonard.

Parties on both sides of the issue frequently invoke the hypothetical economic impact of same-sex marriage, Leonard pointed out, so the influx of real-world data from New York could go a long way toward changing those hypotheticals into concrete facts.

"It becomes less of an experiment the more information we have,'' he added.

The ripple effect of the new law is likely to provide more than just

information, said Goldberg. New York's mobile population means that

the effects of the law will reach literally into other states.

"New Yorkers tend to move about the country quite a lot,'' Goldberg

said. "High numbers of same-sex couples likely to marry here will increase pressure on other states to treat those couples fairly.''

Currently, 39 states have laws defining marriage as between a man and a woman, according to statistics from the National Conference of State Legislatures.

NEW YORK UNIQUE

For states considering how to handle calls for same-sex marriage, Massachusetts -- the first state to legalize it, in 2004 -- has generally served as the reference point, Leonard said. But he noted that New York was different from Massachusetts for two primary

reasons.

First, it has more than three times as many people. Second, New York instituted same-sex marriage through legislation, complete with religious exemptions. Massachusetts, on the other hand, established the right to same-sex marriage in a court ruling.

The significance of that difference cuts both ways, said Michael Dorf, a professor at Cornell Law School who studies the constitutional and social consequences of same-sex marriage in the United States.

When legislation fails to pass, it can serve as evidence of a minority group's political weakness or of widespread prejudice against it, Dorf said. Both are factors courts use under an equal-protection analysis to determine whether to intervene and protect minority rights. The New York legislation's success, in contrast, could lead judges in other states to say, "We don't need to intervene, let the political process work this through,''' Dorf said.

But because courts are also wary to make rulings that are perceived to be too far outside the mainstream, the New York law may begin to tip that balance.

"To the extent that the anti-same-sex marriage argument has been that this is a radical change and incompatible with the country's social mores, the fact that the country's third most populous state has done so shows that it may not be,'' Dorf said.

Regardless of the immediate impact of the law, Dorf said, politics and public opinion on the issue ``are in the course of rapid change.''

"It seems inevitable that we'll have same-sex marriage in most of the states within a decade,'' he said.

(Reporting by Jessica Dye; Editing by Jesse Wegman)

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