Opponents of same-sex marriage worry that allowing two men or two women to wed would radically transform a time-honored institution. But they're way too late on that front. Marriage has already been radically transformed - in a way that makes gay marriage not only inevitable, as Vice President Biden described it in an interview late last year, but also quite logical.

We are near the end of a two-stage revolution in the social understanding and legal definition of marriage. This revolution has overturned the most traditional functions of the institution: to reinforce differences in wealth and power and to establish distinct and unequal roles for men and women under the law.

For millennia, marriage was about property and power rather than love. Parents arranged their children's unions to expand the family labor force, gain well-connected in-laws and seal business deals. Sometimes, to consolidate inheritances, parents prevented their younger children from marrying at all. For many people, marriage was an unavoidable duty. For others, it was a privilege, not a right. Often, servants, slaves and paupers were forbidden to wed.

But a little more than two centuries ago, people began to believe that they had a right to choose their partners on the basis of love rather than having their marriages arranged to suit the interests of parents or the state.

Love, not money, became the main reason for getting married, and more liberal divorce laws logically followed. After all, people reasoned, if love is gone, why persist in the marriage? Divorce rates rose steadily from the 1850s through the 1950s, long before the surge that initially accompanied the broad entry of women into the workforce.

Adopting love as the basis for marriage meant other changes, too, especially greater acceptance of the idea that men and women had a fundamental right to marry, even to people of whom their parents - and society - disapproved. By the 1940s and 1950s, many state courts were repealing laws that prevented particular classes of people from marrying. In 1967, the U.S. Supreme Court ruled that it was unconstitutional for states to prohibit interracial marriage. In 1978, that court struck down a Wisconsin law prohibiting marriage by parents who had not met prior child-support obligations. In 1987, it upheld the right of prison inmates to marry.

But huge as the repercussions of the love revolution were, they did not make same-sex marriage inevitable, because marriage continued to be based on differing roles and rights for husbands and wives: Wives were legally dependent on their husbands and performed specific wifely duties. This was part of what marriage cemented in society, and the reason marriage was between men and women. Only when distinct gender roles ceased to be the organizing principle of marriage - in just the past 40 years - did we start down the road to legalizing unions between two men or two women.

Over the ages, marriage enforced an unequal division of labor, wealth and power between men and women. Traditional English and American law gave the husband sole control over all property that his wife brought to their marriage and any income she earned during it. Husbands had the legal right - and the duty - to impose their will by force. A husband couldn't cede any rights to his wife, said the courts, "because that would presuppose her separate existence," according to Blackstone's Commentaries on the Laws.

By the early 19th century, the old ideas that women needed to be under male authority because they were more prone to sexual passion and religious error than men, and that husbands ruled the home just as monarchs ruled their kingdoms, had given way to a gentler but equally rigid gender ideology. Men were recast as benevolent breadwinners who exercised authority not because they were the patriarchal bosses of the family labor force, but because they were women's natural providers and protectors. Women were frail dependents whose nurturing nature and innate sexual purity predisposed them to sweet submission.

This redefinition of gender allowed 19th-century Americans to reconcile the new ideal of married love with a continued claim that husbands and wives had completely different rights and duties. And in the 20th century, even as the right of individuals to choose their partner became the cultural norm and legal reality, the insistence that marriage united two distinct gender stereotypes became increasingly shrill.

During the 1940s, '50s and '60s, sociologists and psychiatrists remained adamant that marriage required strict adherence to traditional feminine and masculine roles. In 1964, a year after Betty Friedan published "The Feminine Mystique," an article in a journal of the American Medical Association described beating as a "more or less" satisfactory way for an "aggressive, efficient, masculine" wife to "be punished for her castrating activity" and for a husband to "re-establish his masculine identity."

Well into the 1970s, marriage was still legally defined as a union that assigned differing marital rights and obligations according to gender. The husband was responsible for supporting the family financially, but he also got to decide what constituted an adequate level of support, how to dispose of certain kinds of property and where the family would live.