The Equal Protection Clause of the 14th Amendment as a question of constitutional law, is not quite that easy, and is often misinterpreted. For starters, the Supreme Court has made it clear that the guarantee of equal treatment within the equal protection clause does not mean that governments cannot ever treat different people differently. States need not permit children to drive cars, for example, nor must they allow senior citizens to enroll in grammar school. As a basic rule, the Court has said that it is reasonable for states to build categories or classifications into the laws that they pass, and in fact, the "rational basis test" is one of the standards used by the courts to determine whether these classifications are fair. Also known as the Lindsley test, this standard says that if the reasons for treating people differently are reasonable and logically related to the law's purpose, then they are constitutional. Opponents of gay marriage insist that there is a rational basis (usually, they argue, rooted in cultural or religious tradition) for restricting marriage to a relationship between a man and a woman.
But the Court has also held that certain types of laws need to pass a tougher standard than the Lindsley test; certain types of classifications within the law are more suspicious and require closer scrutiny. In particular, the Court has said that America's history of racial oppression requires that all laws employing racial classifications must be more rigorously examined by the courts. Consequently, it is far less likely that these sorts of laws will survive judicial scrutiny. On occasion, the Court has also applied this heightened level of scrutiny to state laws using classifications based not only on race but also on citizenship. And in recent decades, the Court has developed an intermediate standard for evaluating laws employing gender classifications. Statutes that treat men and women differently must be more than merely reasonable, they "must serve important governmental objectives," and the differing treatments of men and women "must be substantially related to achievement of those objectives."
In terms of gay marriage, the critical issue thus becomes the level of scrutiny that laws affecting gays and lesbians should receive. Are gays, like racial minorities, considered a "suspect" class in terms of constitutional law? Does the court rigorously scrutinize laws impacting them? Or do laws that create classifications based on sexual orientation receive a lesser degree of vigilance, like those based on age? Should the courts then apply the lowest level of scrutiny, the rational basis test? Or do they impose an intermediate standard like the one used to examine laws incorporating gender classifications?
The short answer is that, thus far, gays and lesbians have not been considered a suspect class by the Supreme Court; laws impacting them are today subject only to the lowest level of scrutiny. But since the Court has addressed these questions only very recently, it is hard to predict whether this approach will persist too much farther into the future.