The ninth circuit court of appeal's ruling in Perry v Brown (pdf) that California's infamous Proposition 8 unconstitutional is certainly good news. The court's finding that the ban on same-sex marriage enacted by California's voters violates the equal protection clause of the 14th amendment has major potential implications.

Given the importance of the case and the rights and stake, the majority opinion by the liberal lion Judge Stephen Reinhardt might seem surprisingly narrow and cautiously argued. But this reflects the most important audience for Reinhardt's opinion: Justice Anthony Kennedy. Should the supreme court overrule the ninth circuit, the victory won in the lower courts would be not merely hollow, but actively counterproductive. Reinhardt's opinion seems written to maximize the chances that it will be upheld by the supreme court – and he might well succeed.

Given the near-certain votes of Justices Scalia, Thomas, Alito and Chief Justice Roberts to uphold Proposition 8, as with so many others, this case is likely to come down to Justice Kennedy's vote. And while generally Kennedy is an ally of the aforementioned group of conservative Republican appointees, on issues pertaining to gay and lesbian rights, there is some reason for optimism.

When he was nominated for the supreme court by Ronald Reagan, after the Senate rejected Robert Bork, some conservatives objected to Kennedy based on his perceived sympathy to civil rights for gays and lesbians. These objections, fortunately for the country, were well-founded. In Lawrence v Texas, Kennedy authored the opinion for the court overruling the infamous Bowers v Hardwick, which had upheld a Georgia law banning "sodomy" in language rife with contempt for gays and lesbians and their rights. And even before Bowers had been overruled, Kennedy's opinion in Romer v Evans struck down a Colorado initiative that prevented any state institution or agency from providing civil rights protection to gays and lesbians.

Shrewdly, Reinhardt's opinion repeatedly cites Romer and argues that the case presents similar constitutional questions. "Proposition 8," argues Reinhardt, "is remarkably similar to amendment 2" (the Colorado initiative Kennedy held unconstitutional). Kennedy had argued in Romer that amendment 2 was constitutionally invalid because it "withdraws from homosexuals, but no others, specific legal protection". Proposition 8, Reinhardt notes, has the same key characteristic, singling out gays and lesbians for a particular legal disadvantage.

In particular, Reinhardt focuses on two key elements of Proposition 8: the fact that an existing right was taken away, and the fact that same-sex partnerships were legally identical to opposite-sex ones, except that the nomenclature of "marriage" could not be applied to the former. Both of these facts make Proposition 8 analogous to the invidious, irrational discrimination Kennedy held to be unconstitutional in Romer. Denying the term "marriage" to otherwise legally equivalent partnerships can only be justified in order to stigmatize the same-sex relationships. This cannot be consistent with the equal protection of the laws, and the fact that a right was taken away aggravates the effects of the discrimination.

And the fact that only the name "marriage" was denied to same-sex partnerships does not make the exclusion trivial. The term has unique connotations and meaning, and denying it to some citizens in otherwise equal relationships represents the kind of exclusion the 14th amendment was designed to prohibit.

In addition to citing Kennedy's landmark opinion repeatedly, Reinhardt's repeated citing of Romer demonstrates that the court can hold Proposition 8 unconstitutional without radically changing equal protection law. The California initiative, Reinhardt correctly notes, "has no apparent purpose but to impose on gays and lesbians … a majority's private disapproval of them and their relationships." This kind of animus cannot be said to be rationally related to any legitimate state interest. Hence, Proposition 8 is invalid even if one assumes (as the supreme court currently does) that classifications do not require "strict scrutiny" in the way that classifications based on race do. Particularly on such highly visible issues, the court is always more likely to favor a right if it is not required to create new doctrine to do so.

Admittedly, Reinhardt's argument against Proposition 8's constitutionality is not without its downsides. Because of the narrow focus on the specific facts of the California case, if the supreme court were to adopt the reasoning of the ninth circuit, the national implications of the ruling would be unclear. It's likely that lower courts would not immediately interpret such a holding as creating a right to same-sex marriage.

Reinhardt's logic, if the supreme court agrees, might restore same-sex marriage rights in Maine – where they were also taken away by an initiative – but lower courts may be reluctant to apply the right to different contexts. A ruling holding that classifications based on sexual orientation should be subject to strict scrutiny, and that the denial of the fundamental right to marriage cannot possibly survive such scrutiny, would be preferable and more consistent with the values of the US constitution – if it were possible.

Still, Reinhardt's decision to go for a solid base hit rather than a home run represents a sound instinct. Kennedy is much more likely to uphold the ninth circuit if he believes such a ruling to be consistent with his own precedents. Forced to choose between a broad, immediate national right to same-sex marriage and upholding Proposition 8, Kennedy is quite likely to choose the latter. Creating a Bowers-like bad precedent would be a disaster, making it much more difficult to successfully litigate in the future – and also, possibly, allowing states to impose more disabilities on same-sex partnerships.

A ruling that immediately gave California's 37 million residents the right to same-sex marriage and creates a precedent that would likely expand that right to many other, if not all states in the future would still be a major victory. We have to hope that Judge Reinhardt has read Justice Kennedy correctly.