Now that the Supreme Court will decide whether the Constitution forbids limiting marriage to opposite-sex couples, it's time to ponder the rationale for what will likely be a 5-4 decision striking down such laws. (Justice Kennedy will probably assign himself the opinion for a majority of Ginsburg, Breyer, Sotomayor, Kagan, and himself.) Here are some possibilities and their likely consequences.
1) Strict Scrutiny: Equal Protection. The Court may say that sexual orientation is a suspect category of discrimination. Sexual orientation may well be an immutable characteristic but it is not the highly visible trait that makes race a "discrete and insular minority." Nor have gays and lesbians lacked political power in recent decades. Gay rights advocates have won and lost battles, much as any other lobby has done. Of course, there has been a history of prejudice against gays and lesbians, and while that is abating it is not nonexistent. If the Court makes this move it will then have to confront possible state laws that insulate from anti-discrimination statutes people who refuse to provide goods or services to gays and lesbians on account of their religious convictions. The outcome is not obvious. Such cases as Romer suggest that these laws might flunk strict scrutiny, but an argument can be made that the government has a compelling interest in protecting religious convictions, even if that interest is not so high as to invoke the free exercise clause. But the effect of such laws is to single out gays and lesbians for disadvantegeous treatment and I think Romer would apply.
2. Strict Scrutiny: Due Process. The Court might say that marriage is a fundamental right, invoking Loving and other cases. But the assumption of all of those earlier cases was that marriage was an opposite-sex arrangement. Racial barriers to opposite-sex marriage are subject to double scrutiny: race and an absolute bar to what was then understood to be marriage. Of course, society's perceptions are rapidly changing, and this may be the Court's tack. If so, it does raise questions about how much the definition of marriage may (or must) be constitutionally expanded.
3. Intermediate Scrutiny. Applying either or both of equal protection and due process the Court could say that intermediate scrutiny applies. But then the Court would have to confront the circumstances that make sexual orientation or absolute denials of same-sex marriage justified. If there are none, why not just apply strict scrutiny? As Justice Scalia once remarked, "We apply intermediate scrutiny when we think it's a good idea to load the dice."
4. Minimal Scrutiny. The Court might say one (or both) of two things: i) States have no legitimate reason to deny marriage to same-sex couples because the denial is rooted in animus towards gays and lesbians, and ii) even if there is a legitimate reason to limit marriage to opposite-sex couples the limitation is not rationally related to that reason. This works well for such hypothesized reasons as procreation, but it fits uneasily with the claim that it is legitimate for states to refrain from expanding marriage until the larger social consequences of a redefinition of marriage are widely understood. If irrationality means what it has meant in other contexts it's probably rational to have some hesitation about abandoning the ancient custom of marriage. And if this is irrational it opens the door to a more widely applicable voiding of laws on the basis of irrationality. Maybe that's good -- it's certainly libertarian -- but it undermines the long-standing presumption that government actions are usually presumed to be valid. Also, if the Court takes this tack it will have to explain why the Court's deference to state definitions of marriage in Windsor is inapplicable here.
I have no idea what the Court will do, but I'm skeptical that they will apply any form of heightened scrutiny. Prediction is dangerous: "He who lives by the crystal ball must be prepared to eat ground glass."