Boston ? Now that we have celebrated the paper anniversary of same-sex marriage in Massachusetts, may we pause for a moment to admit that the opponents were right: Same-sex marriage is proof of a crisis in traditional marriage.

But gay marriage is not the cause of the crisis, it’s a consequence. The true culprit is, well, Cupid.

What’s Love Got to Do With It? Precisely. Until roughly two centuries ago, the institution of marriage was considered far too important to leave up to the emotions of two people. Marriage was about economics and politics and, more than anything else, about creating new in-laws.

To see how far we’ve strayed from the rule of in-laws to the rule of love, just compare the 16th-century “Romeo and Juliet” to the 21st-century – forgive me, Will – “Shrek 2.” In our modern fairy tales, a father is expected to support a love match even if his daughter marries an ogre. Any mother who interferes is a “Monster-in-Law.”

As Stephanie Coontz points out in her lively new book, “Marriage, A History,” at some point, “love conquered marriage.”

The original support for a love match, she writes, was to “make marriage more secure by getting rid of the cynicism that accompanied mercenary marriage and encouraging couples to place each other first in their affections and loyalties.” From the get-go, social conservatives warned of disaster. If love was the only criteria, people who hadn’t fallen in love might remain single, people who had fallen out of love might demand divorce and even homosexuals could lay a claim to marriage. As Coontz says, they were right. They were just 200 years early.

Gradually, the truly traditional marriage was transformed into the “love-based, male breadwinner marriage” that we now label traditional. It was held in check by the economic dependency of wives, the unreliability of birth control and penalties for having children out of wedlock. In the last 40 years all of this too was changed … by heterosexuals.

“Heterosexuals said marriage should be about love. Heterosexuals claimed the right to decide whether to have children. Heterosexuals said marriage wasn’t about gender roles but about individualized relationships,” says Coontz. “Then gays and lesbians said, ‘Knock, knock. You are talking about me.'”

As Anne and Chad Gifford, former head of Bank of America, wrote this week in The Boston Globe, their son’s wedding “brought home the reality that marriage is about two people who love each other and who desire to commit to a life together.”

Of course, this is a definition of marriage that drives opponents to the ramparts. The backlash that mobilized to ensure that marriage was not between two “people” but between a man and woman also draws a bead, or an arrow, on love. Indeed, running through the tirades is the warning that if marriage is based on love alone, we’ll have marriage between people and their pets, what one wag called petophilia.

But opponents need more than a constitutional amendment banning same-sex marriage to shore up traditional marriage. They need to roll back everything from female independence to divorce, birth control and the ideal of marriage as primarily a personal relationship. While some are happy to do just that, you can’t choose one strand from the fabric of history.

As Coontz notes, what makes marriage more attractive to people today is inexorably entwined with what makes it more vulnerable: “Our high standards for a loving, mutually supportive, lifelong connection between two people.”

The anxiety about the stability of marriage speaks to the longing for a buffer in the winds of individualism. It raises questions that go to the heart of life – who will be there for me? Who can I depend on? The importance of marriage in society says a good deal about the longing for commitment in a transitory world.

James Dobson of Focus on the Family has countered by insisting that “the homosexual agenda is not marriage for gays. It is marriage for no one.” But the nearly 6,200 gay couples who got married in Massachusetts this year suggest quite the opposite. Marriage, with all its vulnerability, remains the gold standard of relationships.

Conventional wisdom tells us that gay marriage mobilized the religious right and helped turn the 2004 election. But here, as they say, the sky didn’t fall, the Red Sox won the World Series … and the public approval rating for same-sex marriage went up from 40 percent to 60 percent.

Over the years, same-sex marriages will be subject to the usual ups and downs, risks and rewards. But now, love has everything to do with it.