The Supreme Court's decision in United States v. Windsor is rightfully hailed as an important milestone in the fight for equality. The decision recognizes the right of gays and lesbians to enter into long-term legal relationships without being deprived of federal financial benefits, as required under the Defense of Marriage Act (DOMA). It thus places those relationships (in states that recognize gay marriage) on a par with the family relationships of heterosexuals. It announces that some gays and lesbians are now welcome to our society.

Why only some gays? Thirty-seven states, with 70 percent of our population, do not allow gays and lesbians to marry. Thus they cannot receive all the federal financial benefits that the rest of the married population is entitled to. What the court did was to carefully craft an opinion that would strike down the federal law while not automatically opening the door to gays in the other states to secure their rights as well.

The Equal Protection Clause in the 14th Amendment of our Constitution prohibits states from depriving persons of the equal protection of the laws. "No State shall ... deny to any person within its jurisdiction the equal protection of the laws." No mention is made whether the federal government is also forbidden to do so. But the Supreme Court has held that whatever the states are forbidden to do under the equal protection clause, the federal government is forbidden from doing under the due process clause of the Fifth Amendment, which does apply to the federal government. The case that established the principle was Bolling v. Sharpe, a case dealing with school segregation in the District of Columbia, decided at the same time as Brown v. Board of Education in 1954. So equal protection principles apply across the board to both governments.

A law or other governmental action will violate the equal protection clause under three circumstances. First, courts examine whenever members of a defined, easily recognizable group (a "suspect" group) has suffered a history of discrimination, cannot leave the group voluntarily, and lack political power on their own to correct legal disabilities imposed on them. The court then asks: Is such a group denied rights that the majority group possesses? If so, such denial will violate the equal protection clause.

Second, if a particular, important, "fundamental" right is denied to anyone (like the right to vote unless you can pay a poll tax), that could violate the equal protection clause.

Third, if the law is simply irrational, that is, it makes no sense or it reveals a desire simply to hurt a defined group, that law would violate the equal protection clause. Thus denying food stamps to unrelated "hippies" living in the same house was struck down because the law revealed a "bare congressional desire to harm a politically unpopular group," as the court held in the Moreno case decided in 1973.

The court could have applied any one of these principles to strike down the Defense of Marriage Act.

First, are the gays a "suspect" group under the equal protection clause? The concept of "suspect categories" was first mentioned in the Korematsu case, decided in 1944, where the court held that it was permissible for the federal authorities to send all Japanese Americans on the West Coast to concentrations camps shortly after Pearl Harbor. Although the court held that whenever all members of a racial group were imposed upon by a law, those impositions were "suspect" and had to be carefully examined. But in the context of a possible Japanese invasion of our nation, the argument went, military necessity justified the exclusion. The concept of a "suspect" group was later applied in conjunction with laws dealing with racial discrimination in the South.

As the law developed, the courts had to determine whether a particular group (1) was a "distinct and insular minority," (2) has suffered a history of legal discrimination, (3) cannot leave the discriminated group, and (4) lacks political power to correct the discrimination against them though the political process. Racial minorities fit that standard exactly, and legal aliens were later added to the "suspect" category. To a lesser extent, females and illegitimate children were also declared to be protected groups so that laws that treated them unfairly should be examined more closely.

A strong argument can be made that gays fit the definition of "suspect" as well. They are a distinct minority that generally hangs out together-they are "insular" in many of the same ways as racial groups are perceived to be. The law certainly discriminated against them, and indeed, until 10 years ago, homosexual conduct was deemed a crime. They cannot suddenly overturn their sexual preference. And as a minority, by themselves they cannot vote to eliminate the laws that injure them.

But to find that gays are a "suspect" group would mean that every law or governmental action that treated them less favorably would fail. The state marriage laws from the 37 other states would immediately become invalid. The court was unwilling to go that far.

How about marriage being a fundamental right? In the famous Loving v. Virginia case, decided in 1967 (more than 10 years after Brown v. Board of Education), the Supreme Court declared that miscegenation laws were unconstitutional. Such laws were based on racial classifications and interfered with "one of the vital personal rights essential to the orderly pursuit of happiness by free men." Well, opponents to gay marriage argue, what about plural marriages or incestuous marriages or marriages of minors? Would they also become legal? Once again, the implications from a broader ruling were perhaps too much for the Supreme Court at this time.

So basically the court took the third road: DOMA was an effort to "harm a politically unpopular group." At least 10 times in the majority opinion, it uses the words "injure," "degrade," "demean," "harm," or "disability." The court noted: "DOMA seeks to injure the very class New York seeks to protect. By doing so it violates basic due process and equal protection principles applicable to the Federal Government.

"This approach limits the reach of the opinion to cover only the particular law before the court, and would have no immediate implications for the future. Of course, civil liberties groups can use much of the analysis in the opinion to press for further enlargement of the right to marry in the other 37 states. Many times in the opinion the court focused on the importance of the right to marry. That right seeks "to protect [its participants] in personhood and dignity." The marriage right involves "a dignity and status of immense import." This status "is a far-reaching legal acknowledgment of the intimate relationship between two people.

"Other courts in the future can easily conclude that these words describe a fundamental right that the other 37 states must recognize. The fight is only beginning.

-Leon Friedman is the Joseph Kushner Distinguished Professor of Civil Liberties Law at Hofstra Law School. He is the co-editor of and contributor to The Justices of the United States Supreme Court: Their Lives and Major Opinions