When Congress passed the Defense of Marriage Act in 1996, same-sex marriage did not exist in the United States. Goodridge v. Department of Public Health, the Massachusetts Supreme Judicial Court’s 4-3 decision finding a right to same-sex marriage in the state constitution, was still seven years in the future.

But the crusade to fundamentally redefine marriage was already in progress, and Washington understood that once gay marriage was legalized anywhere, supporters would go to court to demand that it be recognized everywhere. So Congress enacted the Defense of Marriage Act to make two things clear: First, that no state could be forced to deem a same-sex couple “married” merely because another state did so. And second, that as far as the federal government and federal law were concerned, “marriage” would continue to mean what it had always meant: the union of one man and one woman as husband and wife.

DOMA was not controversial. It was passed by robust bipartisan majorities — 85 senators and 342 representatives — and readily signed by President Bill Clinton. Moreover, it was replicated at the state level almost everywhere: 45 states define marriage as the union of male and female, 30 of them in their constitutions.

But five states and the District of Columbia do allow same-sex couples to marry, and the crusaders’ strategy to redefine marriage by judicial fiat proceeds. Which is why Gay & Lesbian Advocates and Defenders, the legal organization that successfully litigated Goodridge, was in federal court last week, urging a judge to rule that the Defense of Marriage Act unconstitutionally discriminates against gay and lesbian couples.

Attorney Mary Bonauto told US District Judge Joseph Tauro in Boston that by restricting the federal definition of “marriage” to opposite-sex couples, DOMA unfairly relegates married same-sex partners to second-class status. Not only that, she claimed, it negates “the longstanding deference of federal to state law in determining the marital status” of individuals claiming federal benefits.

But neither objection holds water.

The 1996 law does not “single out” same-sex marriages for invidious purposes, as GLAD argues in its brief. DOMA simply does what countless federal laws do: It defines basic legislative terms. Considering how frequently the terms “marriage” and “spouse” are used in federal statutes, rulings, and regulations, it is nonsensical to claim that Congress has no right to clarify their meaning — as nonsensical as claiming that Congress is barred from defining “wetland” or “endangered species” or “disability.”

To be sure, an individual state is free to adopt an irregular definition of marriage — or anything else — for purposes of state law. But it doesn’t have a constitutional right to impose that definition on the rest of the nation. Massachusetts could decide to recognize martial-arts studios as institutions of higher education, and to make them eligible for state-subsidized education loans. Plainly, that anomalous definition of “higher education” would not be binding on the federal student loan program. By the same token, Massachusetts can decide (or be required by its supreme court) to treat same-sex partners as married spouses. But it can hardly insist that its definition of “married spouses” trumps that of the federal government and 45 other states.

Bonauto argued that until DOMA came along, the federal government had always allowed the states to decide who was legally married. “The only thing that changed here,” she told Judge Tauro, “was who was going to marry.”

But the overriding national interest in the fundamental meaning of marriage is something Congress established long before 1996.

In the second half of the 19th century, Congress acted time and again to shut down polygamy, which the Mormon Church at the time encouraged. Beginning with the Morrill Anti-Bigamy Act, signed by President Lincoln in 1862, Congress moved aggressively to establish monogamy as the only lawful form of wedlock in the United States. Eventually Congress went so far as to require voters, jurors, and public officials in Utah to take an antipolygamy oath, and it made a permanent ban on polygamy a condition of Utah statehood.

The Defense of Marriage Act understandably sticks in the craw of those who want marriage to mean something the vast majority of Americans have never accepted. But is the longstanding national definition unconstitutional merely because some people reject it? The federal courts have never said so before; there is no reason for them to say so now.