I've seen some people saying, in the aftermath of today's decision in Lawrence v. Texas, that the Court's decision has somehow foreclosed a constitutional argument for gay marriage, because of the path it took to reaching its result. I think this is wrong, but explaining why will require a mini-lesson in constitutional law. (U.S. lawyers can probably skip this post.)
The path the Court was offered and didn't take is the one that most people thought was more likely: the equal protection argument. The Fourteenth Amendment says that no State shall "deny to any person within its jurisdiction the equal protection of the laws." The argument was that because Texas prohibited sodomy between persons of the same sex, and not persons of the opposite sex, that it denied equal protection of the laws.
I've seen people lamenting that the Court didn't take this path, because accepting that argument could lead towards gay marriage, where its current opinion doesn't. As a corollary, people have pointed out that accepting this argument would be a bigger extension of the Court's jurisprudence.
I think both of these are wrong, or at least not necessarily true. Taking them in reverse order:
In the post I linked to earlier, Professor Balkin states that by not using the equal protection argument, "the Court does not have to hold that gays are a suspect class or that classifications based on sexual orientation are entitled to heightened scrutiny." Which is true. But I think the Court could have invalidated the Texas statute without doing either of those. (Of course, Professor Balkin didn't say it had to do either, but I think a lot of people are assuming that.)
Equal protection jurisprudence is a three-tiered affair. To oversimplify slightly, classifications based on race are subject to strict scrutiny; gender, intermediate scrutiny; everything else, rational basis review. The tougher the scrutiny, the stronger the government interest behind the classification has to be. When the Court struck down Colorado's Amendment 2 (which said, you can't pass of any laws or ordinances that prohibit discrimination on the basis of sexual orientation) in Romer v. Evans, it said it was doing it under rational basis review. Many hoped that the Court would use this case to move sexual orientation into the same tier of scrutiny as gender.
Justice O'Connor's opinion, which concurred only in the result, would have invalidated the statute on rational basis review only, by extending Romer to hold that either moral disapproval or animus could never be a rational basis for a law. Justice O'Connor felt that the Texas law was only motivated by one or both of those, thus making it unconstitutional. (Note that O'Connor joined the Bowers majority opinion.) I think this would have been an entirely plausible majority opinion, and probably the one most people expected to see. So invalidating Texas's law could have been done on rational basis review, and a decision on equal protection grounds wouldn't necessarily have been a big step past Romer. And I think it's a tossup whether prohibitions on gay marriage would pass rational basis scrutiny at the Court.
The path the Court did take is what's known as the substantive due process argument. The Fourteenth Amendment says in part that no State shall "deprive any person of life, liberty, or property, without due process of law." The Court has defined "liberty" to include certain rights clustered around ideas of family, sexuality, reproduction, and privacy; these get labeled "fundamental rights." So, for instance, there's a fundamental right to direct the raising of your children, to access to contraception, and to abortion. Obviously these rights are subject to government regulation, but the government has a heavier burden to justify that regulation.
After today, there is a fundamental right to engage in private, consensual sexual conduct, no matter what gender your partner is. This stems from the recognition that such conduct "can be but one element in a personal bond that is more enduring" (slip opinion at 6) and that the ability to choose these personal bonds is central to the Fourteenth Amendment's liberty (slip opinion at 13).
Here's the thing. The Court has previously established that the right to marriage is also a fundamental right (in Zablocki v. Redhail, among others). And the Court's rationale for protecting private sexual conduct applies to marriage just as well—personal bonds, yes? So I think it's a small logical leap to find that marriage is a fundamental right, no matter what gender your would-be spouse is. (Obviously it's a bigger political and social leap for the Court.) Then we're back at the question of whether the government's interest in regulating that right is sufficient; and for fundamental rights, then the interest has to be quite strong: a compelling interest, limited by narrowly-drawn laws.
So, the lack of an equal protection argument doesn't mean that the constitutional argument for gay marriage is dead. On the contrary: while the standards of review tend to be squishy in practice (many people think Romer is not actually rational basis review, for instance), on its face, fundamental rights scrutiny is tougher than intermediate scrutiny, which is almost certainly the highest that sexual orientation would receive.
Do I expect to see the Supreme Court hold that gay marriage is constitutionally required? Yes. Do I expect to see it soon? Not necessarily. I don't know what rationale will be used, but it could be either equal protection or substantive due process; Lawrence v. Texas doesn't rule out either.
[Edited to fix one case name and add another.]