The demand that society legally recognize same-sex marriages is often called revolutionary.  And so it is, but not in the way most people assume. The reason it is revolutionary is not because traditional marriage has always been "one man/one woman." Nor is there anything historically unprecedented in societies accepting and validating same-sex relationships.

But the kind of same-sex relationships envisioned by most gays and lesbians who seek to wed is indeed unprecedented.  The demand that individuals should be able to choose their partner solely on the basis of love, sexual attraction, and mutual interests is indeed a huge challenge to traditional marriage.

And it was heterosexuals who pioneered this revolution. Gays and lesbians have simply asked to join it.

The kind of marriage that we cherish today—the kind that opponents of same-sex marriage believe they are defending against desecration by gays and lesbians, and the kind that has inspired so many gays and lesbians who were once suspicious of the institution to now clamor for inclusion—represents a radical break with thousands of years of tradition. If modern Americans fully understood what traditional marriage actually entailed, few, we can be sure, would want any part of it.

The Many Types of Marriage

There is nothing unusual about forms of marriage that involve something other than one man and one woman.

In a majority of cultures throughout history, the most favored form of marriage was polygyny—one man and multiple women. We're not just talking about exotic island cultures or lost tribes in the African jungle. In 70 percent of more than 1,000 societies described in the Human Relations Area Files, polygyny is the preferred (though not necessarily the more frequent) form of marriage.

Polygyny is the family structure most often mentioned in the first five books of the Old Testament.  It was common throughout ancient India, the Middle East, Africa, China, and many kingdoms in South America. The upper classes in several regions of what is now Europe also practiced it prior to the 7th century. A more recent study of almost 400 societies, which excluded smaller and less well-known samples, found that 60 percent of these contained significant numbers of polygynous marriages.

Polyandry—one woman and many men—has also been found in some societies. In Tibet and parts of India, Kashmir, and Nepal, a woman may be married to two or more brothers, none of whom can claim exclusive sexual rights to her.

Some societies have recognized marriages that didn't even unite two live human beings. In China and the Sudan, when two sets of parents wanted to forge closer family ties through marriage, but no living spouse was available, they sometimes married off a child to the "ghost" of a dead son or daughter of the other family. Among the Bella Coola and Kwakiutl native societies of the Pacific Northwest, when two families wished to establish the trading ties that went with becoming in-laws but didn't have two sets of marriageable children available, they might draw up a marriage contract between a son or daughter and a dog belonging to the desired in-laws.

Nor is there anything revolutionary about cultures accepting same-sex relationships. In fact, the majority of cultures surveyed by anthropologists have accepted same-sex relationships under certain circumstances.

In ancient Greece, such relationships were regarded as purer and deeper than heterosexual bonds. The Greek philosopher Plato declared that love was a wonderful emotion, leading men to behave in honorable ways. But, he quickly explained, he was referring not to the love of women, "such as the meaner men feel," but to the love of a man for another man.

The Christian tradition was more condemnatory toward same-sex relationships, but on the other hand, early Christianity wasn't too keen on heterosexual relationships either. St. Paul maintained that getting married was better than being consumed with passion and giving in to sin, but he argued that staying single and celibate was the best way to serve the Lord. In the medieval European hierarchy of female virtue, the unmarried virgin came in first. The widow, safely delivered from the corruptions of the flesh, came second. And the wife occupied the lowest rung of respectable womanhood.

In the modern industrial world, the United States remains an anomaly in its intolerance toward same-sex relationships. In 2002, an international poll found that 42 percent of Americans believed that homosexuality was morally wrong, compared to just 5 percent of Spaniards, 13 percent of the French, and 16 percent of Italians.

In December 2008, 66 member states of the United Nations signed a statement calling for the decriminalization of homosexuality worldwide. The United States was the only major Western nation that refused to sign. Today, countries as diverse as the Czech Republic, Spain, Norway, South Africa, Australia, Canada, and Croatia permit same-sex domestic partnerships or marriage, with Taiwan and Nepal soon to sign on.

Acceptance of same-sex marriage is rarer in the historical record, but it too exists. There are at least 30 societies in Africa where a woman could traditionally marry another woman and be counted as a "female husband." In fact, among the Lovedu, the queen was required to marry a woman rather than a man. In these marriages, any children that the wife brought to the marriage or bore within it were counted as part of the descent line of the female husband.

Numerous African and Native American societies have recognized male-male marriages. And many cultures of the past have allowed adult males to take "boy-wives," usually on a temporary basis.

But same-sex marriages of the past—like heterosexual marriages of the past—were based upon the idea that marriage required a union between individuals who played very different gender-associated roles, and had different duties, rights, and power.

Female husbands in history were almost always women who for one reason or another had taken on the social roles or possessed the economic resources usually associated with men. Thus a woman who had amassed cattle might marry another woman, both to ensure that she could exercise the legal and social authority traditionally granted to husbands and to pass on property to the children born to her wife. She also commanded the loyalty of these children and reaped the benefits of marrying them off.

Similarly, in male same-sex marriages of the past, one of the men was almost invariably expected to do work traditionally assigned to women in that culture, while the partner usually played the traditional male role. And in societies where men married "boy-wives," their relationships were not based on the kind of equality that is envisioned by most contemporary same-sex couples who seek to marry.

The boy-wife performed the social, cultural, and physical roles normally assigned to women. A boy-wife was expected to be passive, both in his social behavior and in the sex act. Sometimes becoming a boy-wife was a temporary stage where a young man learned, by being the recipient, how men were supposed to treat women.

But these same-sex marriages of the past were not an alternative to heterosexual marriage. They were an exact replica of it.

The contemporary vision of same-sex marriage, which rejects rigid gender roles and power hierarchies, is a revolutionary approach to marriage. But it is an extension of a revolution that has occurred in heterosexual marriage.

Marriage, Money, and Control

For thousands of years, heterosexual marriage was an institution designed primarily to regulate property rights and political privileges, as well as to enforce rigid age and gender hierarchies. Economically, marriage filled the role that markets and banks do today. It organized transfers of property and gave individuals access to new workers for the family business or farm.

Among the upper classes, marriage was the main way of effecting business mergers, forging military coalitions, and finalizing peace treaties. Royals and nobles sought partners whose lineage could bolster their claim to social status or political authority, whose property holdings would enlarge their power, and whose in-laws would be useful allies.

Marriage was also a central economic and political institution for the European middle classes. They too sought "connected" in-laws. Until the late eighteenth century, the dowry a man received at marriage was often the biggest single piece of cash or movable goods he would acquire in his lifetime, with the result that most men were more interested in the dowry than the daughter.

A woman's parents invested in her dowry to give her economic security in the same way that today's parents invest a college education fund, and they were as insistent that she marry the man they chose as most parents today are about their daughters completing their high school education . The "widow's third" that marriage guaranteed a woman on her husband's death throughout most of Europe was the closest thing to social security she would ever see, which meant that women too, when they had a choice at all, were usually ruled less by sentiment than practicality in choosing their marriage partner.

Even among the lower classes, marriage partners were more often chosen for what they could contribute to the family livelihood—particularly in terms of their work ethic, strength, skills, and robust health—rather than for such a frivolous reason as love. Love was a nice bonus if it could be combined with more practical gains from marriage, but it was not essential. Peasants looked for a partner whose family had adjoining plots of land or good connections with the local authorities. Farmers and craftsmen wanted to find someone who could contribute to the family enterprise, which is why the children of bakers, leatherworkers, and other craftsmen so often married each other.

Of course, throughout history many young people dreamed of marrying someone they could love, but most understood that practical considerations came first. And if they balked, their parents had many ways to force them to follow the family's interests rather than their foolish hearts. It is no accident that most love stories of the past ended in tragedy.

Few societies in history believed that parents had an obligation to let young people freely choose their own mates. Until the late 18th century, parents took for granted their right to arrange their children's marriages and even, in many regions, to dissolve a marriage made without their permission.