It was fitting that when the Supreme Court heard arguments on same-sex marriage bans this Tuesday, a healthy part of the discussion revolved around the "definition of marriage" — not only what that definition is, but what it has been, what it might be, and whether the definition we have today should prevail for all time. To be honest, I've always found it a bit curious that same-sex marriage opponents wield "You want to change the definition of marriage!" like a trump card, as though changing the definition of marriage is so self-evidently horrifying that everyone agrees it should never be done.

But like so many of the arguments that opponents of same-sex marriage have made over the years, this one withers at any careful examination. We've known for some time that the power of competing arguments has little to do with how the Court will ultimately rule (as so often happens, this case will probably turn on the fickle whims of Anthony Kennedy). Nevertheless, the arguments revealed starkly differing views not just on what the definition of marriage is, but what it means to change.

From the outset, it was clear that for the conservative justices, a change in definition was at a minimum something to be feared. In his first question, Chief Justice Roberts reported to advocate Mary Bonauto on his dictionary research: "Every definition that I looked up," he said, "prior to about a dozen years ago, defined marriage as a unity between a man and a woman as husband and wife. Obviously, if you succeed, that core definition will no longer be operable." He then objected to Bonauto asserting that gay people wanted to "join" the institution: "you're not seeking to join the institution, you're seeking to change what the institution is," as though that was obviously problematic.

When it was his turn to speak, John Bursch, arguing in opposition to same-sex marriage, began his presentation by saying, "This case isn't about how to define marriage. It's about who gets to decide that question." But it turned out otherwise. When pressed by the justices to explain how gay people getting married would harm heterosexual marriages and the state's interest in having parents raise their children, Bursch said, "there's harm if you change the definition because, in people's minds, if marriage and creating children don't have anything to do with each other, then what do you expect? You expect more children outside of marriage." Later, Bursch came back to the definition question again: "Our point is that when you change something as fundamental as the marriage definition, as Chief Justice Roberts was saying, the dictionary definition which has existed for millennia, and you apply that over generations, that those changes matter."

Bursch had very little success describing how those changes would "matter" in a negative way; when he tried to argue that the knowledge that gay couples are marrying would make straight couples stop having children, the liberal justices feasted on him like a pride of lions tearing apart a wildebeest. But he and the conservative justices were really arguing for a position that their opponents don't dispute: Of course we're talking about changing the definition of marriage.

The reason is that the definition in existing law is no longer in line with our values. That's what we've done many times before, despite all the misinformed talk about the current definition having been enshrined for millennia. When we decided that we no longer wanted the definition of marriage to be a union between one man and as many women as he pleased, we changed the definition. The definition of marriage in this country at one time meant a union between a white man and a white woman; when we could no longer tolerate that, we changed the definition, just as we did later to include interracial marriages. A marriage no longer means a woman becomes her husband's property, which was also part of the definition. (Justice Ginsburg pointed that out when she said that in an earlier time, "Marriage was a relationship of a dominant male to a subordinate female. That ended as a result of this Court's decision in 1982 when Louisiana's Head and Master Rule was struck down.")

So the question isn't "are we changing the definition of marriage?", because we are. The question is whether our contemporary values, and a Constitution that enshrines individual liberty and equal treatment under the law, demand this particular change. That's also the answer to the inevitable question from conservatives: "If we allow gays to marry, what's to stop us from allowing a man to marry his sister or a woman to marry her dog?" Well, what's stopping us now? It isn't because we're locked into a particular definition that was set thousands of years ago and must remain for all time, it's because we've decided to draw a line that doesn't include incestuous or human/canine unions. The fact that we might move the location of that line doesn't disallow us from drawing any line at all, any more than it did when we outlawed polygamy.

If a few decades ago you had asked most people about the "definition of marriage," they wouldn't have thought you were talking about the law and the constitution; the definition was so taken for granted that it didn't seem like a definition at all. And I think this is a big part of why the conservatives who oppose marriage equality find this whole debate so unsettling. Obviously, there is a broad constellation of reasons why many conservatives, particularly those who are older, would be opposed to same-sex marriage. But the idea of change is among the most powerful.

They see change all around them, very little of it to their liking — a country where people speak languages they don't understand, where the young are absorbed in technologies they find intimidating, where it's OK to say "shit" on television, where the world of their youth, when everything was simple and pure, drifts farther into the hazy mists of history with each passing day. And when something as basic as marriage being between a man and a woman is no longer taken for granted but becomes a matter of debate, it doesn't matter whether you can come up with a persuasive set of negative consequences that will result from a change. Some kinds of change are bad, just because they are.

Not everyone who opposes marriage equality feels this way, and I'm sure some activists on the right have genuinely convinced themselves that there's a compelling practical reason to take the position they do, because if gay people are allowed to marry, straight parents will start divorcing each other in droves and leaving their children behind in highway rest stops. But when so many of the arguments traditionalists used to use against allowing gay people to marry had to be discarded along the way, the simple aversion to change — we have to define it this way because this is the way we've defined it in the past — may be all they're left with.