Stephanie Coontz teaches marriage and family history at The Evergreen State College in Olympia, Wash. She is the author of "Marriage, A History" and, most recently, "A Strange Stirring: The Feminine Mystique and American Women at the Dawn of the 1960s."

For the past several years, we've heard predictions that legalizing same-sex unions will overturn marriage as the Western world has known it for 5,000 years, destroying a tried-and-true institution. But history reveals that marriage has been an evolving arrangement throughout the centuries, remaining relevant only by adjusting to changing social norms and values.

The well-nigh unstoppable drive to legalize same-sex marriage throughout the world -- seen most recently in New York on Friday night -- is a product of innovations in marriage that heterosexuals have been implementing for the past millennium. And no state or country that has legalized same-sex marriage has experienced a deterioration in heterosexual marriage since doing so. In fact, in most of those places, the rise in divorce rates and the fall in marriage rates that had characterized the 30 years before legalization of same-sex unions have since come to an end.

For thousands of years, same-sex marriage was indeed incompatible with the beliefs and practices of heterosexual marriage in most of the Western world. Marriage in the old days was not based on love, sexual compatibility, or equality between husband and wife. It was about sealing political alliances and business deals, gaining well-placed in-laws, making the family workplace more productive through the labor of one's children, and making sure that only legitimate children, born to a parentally sanctioned match, had any claim on the family's property.

Parents could and did prohibit or dissolve a match made without their approval. A "love child" had no right to parental support. A man whose wife failed to bear him an heir could divorce her or even take a second wife.

Ironically, the Catholic Church put the first nail in the coffin of traditional marriage. To implement Jesus' prohibition of the age-old customs of divorce and polygamy, the early church ruled that the validity of a marriage did not depend on the ability of a couple to procreate. If you were capable of having children, church leaders declared, you must not try to prevent conception. But a couple incapable of procreating had every right to marry -- and no excuse to divorce.

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Even more radically, the medieval church conceded that a couple's love could outweigh the disapproval of parents, community, and the church itself. If a couple claimed to have exchanged vows of consent in private, even without anyone's approval, the church ruled, parents could no longer nullify the match. In 1215, the church defined a "licit" marriage as one where the couple posted public notices of their intent to marry in advance (implying community approval), the bride had a dowry (implying parental approval), and the wedding took place in church. But tellingly, the church insisted that an "illicit" marriage was still valid. The couple was entitled to full church membership and to have their children baptized in church. No one, said the church, had the right to grant or impose a divorce.

Nevertheless, marrying for love instead of for economic and political calculation remained rare and socially disapproved until the late 18th century, when radicals such as America's founding fathers insisted that people had a right to "the pursuit of happiness." Almost immediately, youths began to defy their parents and marry -- or not marry -- as they wished. Individuals also began to demand the right to divorce rather than remain in an unhappy marriage, and rates of divorce -- which had been illegal in most Western societies in the early modern era -- rose steadily throughout the 19th century.

The next step in overturning traditional marriage came in the early 20th century, when Americans began to believe that a legitimate marriage depended not just on a wife's duty to have sex with her husband, whether she liked it or not, but on mutual sexual attraction and compatibility. Then, between the 1930s and the 1960s, reformers overturned laws outlawing contraceptives, giving married couples the right to prevent procreation if they chose. In the 1960s and 1970s, states repealed laws that labeled assisted reproduction a violation of the marriage contract. This meant that couples biologically incapable of procreating gained the right to become parents.

And in overturning laws against interracial marriage in 1967 -- a practice 70 percent of Americans condemned in that era -- the Supreme Court reaffirmed that community disapproval cannot override the right to marry.

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The final nail in the coffin of traditional heterosexual marriage came in the 1970s, when courts acceded to the growing demands for gender equality in marriage by abolishing legal statutes assigning differential rights and duties to husbands and wives.

These were the changes that revolutionized marriage. Traditional heterosexual marriage denied the primacy of love and free choice in mate selection, sanctioned marital rape, prohibited contraception and assisted reproduction, and assigned husbands and wives distinct, unequal roles in marriage. It was heterosexuals who transformed this historical institution into something completely different.

No wonder so many gays and lesbians suddenly began to demand the right to join an institution that is now so much more relevant to them than in the past. And no wonder a majority of heterosexual Americans have gradually come to support that demand. According to the "Constructing the Family" surveys by Indiana University's Brian Powell, by 2010 a majority of Americans -- 52 percent -- approved of same-sex marriage, up from 41 percent just seven years earlier. An ABC News/Washington Post poll this past March found a similar shift.

No wonder as well that modern heterosexual marriage has not been undermined by same-sex marriage. The acceleration of divorce in America began well before the movement for same-sex unions gained traction. In fact, divorce has declined since that time, falling from 22.6 divorces per 1,000 marriages in 1979 to 16.4 in 2009. States that favor same-sex marriage have lower divorce rates than most states that disapprove it.

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An international study by University of Massachusetts at Amherst's Lee Badgett, author of "When Gay People Get Married," reveals similar trends. In most of the European and Nordic countries that extended recognition to same-sex couples in the past 15 years, divorce rates are lower today than 20 years ago, and heterosexual marriage has actually ticked up. Skepticism about the institution of marriage rose more in countries that failed to grant any legal status to same-sex couples than in countries that did.

New York will soon become the sixth and largest state where same-sex couples can wed. As its citizens adjust to this latest shift, they will come to see that most of the really revolutionary changes to marriage have already occurred. Acceptance of same-sex marriage is a symptom -- not a cause -- of those changes.