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.