It's fun to mock Andrew Sullivan's silly essay in Newsweek/The Daily Beast declaring Barack Obama the first gay president. But I'll give the outlet credit for also publishing a much better article by the historian Stephanie Coontz, arguing that the rise of gay unions is an outgrowth of a radical shift in straight unions:

For millennia, marriage was about property and power rather than mutual attraction. It was a way of forging political alliances, sealing business deals, and expanding the family labor force. For many people, marriage was an unavoidable duty. For others, it was a privilege, not a right. Servants, slaves, and paupers were often forbidden to wed, and even among the rich, families sometimes sent a younger child to a nunnery or monastery rather than allow them to marry and break up the family's landholding.

The redefinition of traditional marriage began about 250 years ago, when Westerners began to allow young people to choose their partners on the basis of love rather than having their marriages arranged to suit the interests of their parents. Then, just 100 years ago, courts and public opinion began to extend that right even to marriages that parents and society disapproved.

In the 1940s and 1950s, many states repealed laws that prevented particular classes of people -- including those with tuberculosis and "the feeble-minded" -- from marrying. In 1967 the U.S. Supreme Court ruled it unconstitutional for states to prohibit interracial marriage. In 1987 it upheld the right of prison inmates to marry....

But the most important cultural change that has increased support for same-sex marriage is the equality revolution within heterosexual marriage.

For most of history, the subordination of wives to husbands was enforced by law and custom. As late as the 1960s, American legal codes assigned differing marital rights and obligations by gender....Between the 1970s and 1990s, however, most Americans came to view marriage as a relationship between two individuals who were free to organize their partnership on the basis of personal inclination rather than preassigned gender roles. Legal codes were rewritten to be gender neutral, and men's and women's activities both at home and work began to converge....The collapse of rigid gender expectations and norms has fostered the expectation that marriage should be an individually negotiated relationship between equals, replacing the older notion of marriage as a prefabricated institution where traditional roles and rules must be obeyed.

Add the growing acceptance of childless heterosexual unions -- and a growing assortment of ways for gays to raise children -- and it becomes much harder to argue against allowing couples of the same sex to marry. The more historically aware social conservatives understand this, which is why some of them write essays with titles like "Why Homosexuals Want What Marriage Has Now Become."

What has modern marriage become? Here's Coontz again:

Marriage is now more optional than in the past, and people are far less willing to remain in a marriage that doesn’t feel fair, loving, and mutually respectful. On the other hand, as a result of these changes, many marriages have become more fulfilling and mutually beneficial than ever before.

Domestic violence rates have plummeted over the past 30 years, dropping by 50 percent since 1980. The divorce rate, which rose sharply in the 1960s and 1970s, has been falling since its peak in 1981, and it has fallen the most for educated couples, who are the most likely to mix and match traditional gender behaviors.

Elsewhere in Reason: More marital history from Thaddeus Russell and from yours truly. I commented on that "Why Homosexuals Want..." essay here.

Elsewhere not in Reason: Roderick Long reminds us: "In the traditional meaning of 'marriage,' then, there are no married couples in the United States today."