Many equal protection challenges to the recent onslaught of antitransgender legislation ask courts to determine the constitutional limits of the state’s ability to define sex. In these cases, transgender plaintiffs argue that the state violates constitutional guarantees of sex equality when its definition of sex coercively assigns them to a sex category against their will. The canon of constitutional sex discrimination, however, does not directly raise or answer definitional questions of sex. The canonical cases addressed the state’s ability to treat men differently from women—not the state’s ability to define “men” and “women.” This difference between the canonical cases and what this Article calls “sex-defining” cases does not necessitate any monumental shifts in equal protection doctrine, but it does require courts to tweak their intermediate scrutiny analyses. The canonical cases tested the fit between the state’s important interests and the law’s differential treatment of men and women, but sex-defining cases require courts to test the fit between the state’s important interests and the law’s definition of sex. In most sex-defining cases, however, courts ignore this essential question. This failure has produced pro-trans decisions that are correct in their conclusions but flawed in their reasoning. As the likelihood of a grant of certiorari in one of these sexdefining cases rises, so do the stakes of these mistakes.

This Article uses the pro-trans bathroom cases to illustrate what can go wrong when courts fail to examine the connection between the state’s definition of sex and its proffered justifications for the law. These decisions deem trans-exclusionary bathroom rules unconstitutional for reasons that have little to do with the state’s definition of sex, and instead, employ rationales that are fact-bound, narrow, and doctrinally questionable. Anti-trans courts, therefore, can and have upheld these laws with superficial and circular reasoning, without rebutting counter-analyses from pro-trans decisions. The second half of this Article is prescriptive and urges courts to adopt a contextual approach to sex when analyzing challenges to sex-defining laws. It makes clear how this approach flows directly from equal protection doctrine and how it lends itself to more doctrinally disciplined and normatively sound conclusions in sex-defining cases.