The Equal Protection Clause of the Fourteenth Amendment, which provides that no state shall deny any person "the equal protection of the laws," is the relevant constitutional text. But what does it mean?
A simple interpretation might suggest that the government may never treat people differently. But that is an implausible understanding of the text. All laws treat people differently. Speed limit laws treat people who drive 75 miles per hour differently than those who drive 45 miles per hour. People who have gone to medical school can practice medicine; others cannot. Citizens can vote; aliens cannot. In-state college students pay a lower tuition than out-of-state college students. People over 65 receive certain benefits that are not available to people under 65. And so on.
Surely, the Equal Protection Clause cannot mean that all such laws are unconstitutional. Recognizing this, the Supreme Court has held that most laws that treat some people differently from others are constitutional if the difference in treatment rationally furthers a legitimate government interest. As illustrated by the examples noted above, almost all laws pass this test.
But this does not exhaust the meaning of the Equal Protection Clause. The primary goal of the Clause, which was enacted in the wake of the Civil War, was to prohibit laws that discriminate against African-Americans. To effectuate that purpose, the Supreme Court has held that laws that discriminate against African Americans violate the Equal Protection Clause unless they pass "heightened scrutiny" -- that is, unless the discrimination is necessary to further an important government interest.
But is heightened scrutiny limited only to laws that discriminate against African Americans? That would be odd, because the text says nothing about limiting its core protection to African Americans. Rather, the text is open-ended, and it is therefore reasonable to assume that discrimination against African Americans was seen not as a singular problem, but as a paradigmatic one. That is, it is the paradigm of a certain type of discrimination that is especially problematic under the Equal Protection Clause.
Applying this understanding, the Supreme Court has concluded that the proper application of the Clause requires the use of heightened scrutiny to test the constitutionality of laws that discriminate against African-Americans or that discriminate against other groups in society that are similar to African Americans for purposes of the Equal Protection Clause.
What, though, does it mean to be "similar to African Americans for purposes of the Equal Protection Clause"? The Supreme Court has looked to several factors. First, it considers whether the group has been subjected to a history of discrimination. This is relevant both because such a history suggests that there may be prejudices at work in society that can taint the fairness of the political process, and because it is particularly unfair to heap additional burdens on groups that have been systematically discriminated against in the past.
Second, the Court considers whether the group can effectively protect itself in the political process. If a group does not have that ability, then it is especially vulnerable to the pernicious effects of prejudice and intolerance.
Third, the Court considers whether the group is objectively different in some meaningful way that would logically justify treating its members differently than others. For example, it is sensible to treat people born with severe learning disabilities differently in some ways than others, but it is not sensible to assume that race is relevant to an individual's capacity to function fully in society.
Finally, the Court considers whether the group's status is immutable. That is, African Americans cannot change their race. Therefore, laws that discriminate against African Americans are particularly unjust, because it is unfair to disadvantage people for characteristics that are largely beyond their control.
Considering all these factors, the Supreme Court has concluded, for example, that laws that discriminate against ethnic minorities and women are sufficiently similar to laws that discriminate against African Americans to justify testing them by heightened scrutiny.
What, though, of gays and lesbian? How do they fare under this analysis? The first three criteria seem clear. There can be no doubt that gays and lesbians have been subjected to a long and often tragic history of discrimination -- even to the point that they were declared to be criminals. They are certainly a political minority, even more so than African Americans and women, and historically they have been particularly powerless politically because they were forced into the closet and were therefore effectively unable to represent their interests in the political process. And there is no reason to believe that gays and lesbians are any less able to function well in society than anyone else -- even to the point that they are now permitted to serve openly in the military.
The only criteria on which there is any question is the fourth, but there is a general consensus today that one's sexual orientation is not a matter of choice. Although there are those who dispute this proposition, the great weight of the evidence cuts the other way. If you are a heterosexual, imagine if you suddenly had to lead your life as a homosexual. All of your instincts would cut strongly in the opposite direction. You might be able to force yourself to engage in sex with people of the same sex, but it would seem wholly unnatural and, more importantly, you would continue (secretly) to be attracted to persons of the opposite sex, even if you could no longer legally act on those attractions. This is pretty much what sexual orientation means, and in its deepest sense the orientation itself seems to be beyond one's own control. One can (perhaps) change one's conduct, but not one's orientation.
Thus, like laws that discriminate against African Americans, ethnic minorities and women, laws that discriminate against gays and lesbians must be tested by heightened scrutiny under the Equal Protection Clause. And, as a practical matter, tested by that standard, it is difficult to think of any interest furthered by the Defense of Marriage Act that would enable that misguided law to pass constitutional muster.