The formalist argument is that such bans classify on the basis of sex in a very basic way: In states where a man cannot marry a man, he is deprived of this right by virtue of his sex. That is, were the man a woman, he would have the right to marry his mate.

But Berzon’s opinion pushed past that formalist logic to make a stronger argument that is both nuanced and profoundly true: Laws that discriminate against same-sex couples are, on some level, based in sex stereotypes about what constitutes a “real man” and a “real woman.” The Supreme Court has recognized that laws based on sex stereotypes amount to sex discrimination in violation of Equal Protection. Most famously, in the 1991 case United States v. Virginia, the Court forced the Virginia Military Institute to admit women despite the school’s belief that female candidates were too naturally “cooperative” for its antagonistic methods.

Berzon sees a different shade of sex discrimination in marriage bans. By refusing to recognize the rights of a man who loves a man or a woman who loves a woman, these laws police the boundaries of masculinity and femininity, rejecting identities that challenge traditional gender roles by being “too effeminate” or “too butch.” This reasoning seems intuitively right. Part of homophobia is undeniably sex stereotyping. Long before marriage equality was hip, Adrienne Rich spoke of “compulsory heterosexuality”—part of being a good woman is loving men.

The sex-discrimination argument, if picked up by the Supreme Court, would also have some immediate and sweeping implications for queer people. LGB persons could get a whole host of rights and protections in one fell swoop, without waiting on Congress to pass meaningful legislation.

Take the workplace, for example. The ban on sex discrimination in Title VII of the Civil Rights Act of 1964 has rarely been extended to apply to queer employees (with the exception of Terveer v. Billington, a D.C. District Court decision that reasoned along the same lines as Berzon). The federal Employment Nondiscrimination Act (ENDA) would plug this hole. But efforts to pass the law have stalled, and many organizations that were initially proponents of the legislation have withdrawn their support in the wake of the Hobby Lobby decision, because of the bill’s religious exemptions: Now riddled with loopholes, ENDA would allow discrimination to continue under a façade of religiosity. Were the Supreme Court to adopt this sex-stereotyping argument to effectuate marriage equality, however, it is possible that there would be no need for legislation like ENDA to give voice to arguments about sexual-orientation discrimination in courts, because constitutional and statutory sex-discrimination provisions (including Title VII) could cover sexual-orientation discrimination as well.