As the debate over same-sex marriage has proceeded, one of the arguments you hear most often from those opposed to marriage equality is that there is this thing called "traditional marriage" that has been exactly the same for thousands of years, and if we "change the definition of marriage" to include gay people, well then things are really going to get crazy. There'll be no more rationale for keeping siblings from marrying, or keeping a guy from marrying his dog, or keeping a fish from marrying a toaster. What I don't often hear liberals say in response is: Yes, we are changing the definition of marriage. And that's OK.
I think it's because advocates of marriage equality understand that change can often be scary, so the impulse is to say, don't worry, this really isn't any big deal unless you're gay. There's no reason why your extremely, adamantly heterosexual marriage will be affected one way or another if your gay neighbors tie the knot. That happens to be true, and one of the things that distinguishes this particular redefinition of marriage is that it doesn't have any practical impact, real or potential, on most of the participants in the institution. That makes it unlike many of the previous redefinitions. For instance, when we changed the definition of marriage so that it no longer meant that a woman went from being her father's property to being her husband's property, every marriage was implicated. And that's just one of the many redefinitions marriage has undergone even in relatively recent history. So it's good to remember just how radically marriage has already changed.
As historian Stephanie Coontz explains in a helpful piece at the Daily Beast, same-sex marriage may be the inevitable result of all these redefinitions, since they all have been moving in the same direction, what she calls the "equality revolution." When you give individuals, particularly those who were previously without power, more autonomy within marriage, the institution just keeps opening up:
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.
The result is a paradox. 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.
The growing acceptance of same-sex marriage is the result of these profound changes in heterosexual marriage. It’s not just the president’s views on marriage that have evolved. Marriage itself has evolved in ways that make it harder to justify excluding same-sex couples from its benefits and obligations.
So even if conservatives are wrong that the institution of marriage has been the same for thousands of years, they're right in their belief that the institution as they understand it is under threat. The fact is that today, marriage is less bound by strict rules and traditions—in other words, less institutional—than ever before. I'd bet this is the case even for many of the conservatives who argue publicly that the institution's traditions need to be maintained (this is hardly a new irony; I was always puzzled by the picture of an ambitious, powerful career woman like Phyllis Schlafly barnstorming the country to convince people that women should stay in the home). Even if some of the male ones have a wife who doesn't work outside the home, I promise you their marriages are much more relationships between equals than they would have been 50 years ago, not to mention what they would have been 100 years ago.
Conservatives may hold off marriage equality for a while, at least in many places. But marriages are already more equal, and there's nothing they can do about that, even if they wanted to. Which I doubt they do.