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Alliance Defending Freedom, the conservative Christian legal organization that litigated Dobbs, is going after a new and possibly surprising target. ADF, having helped undo the right to choose an abortion, now wants to dismantle the sex-equality cases that are Justice Ruth Bader Ginsburg’s greatest legacy—and have protected Americans against discrimination on the basis of sex, sexual orientation, and gender identity. The Supreme Court’s decision last term in West Virginia v. B.P.J. allowing states to ban trans athletes is the latest under-the-radar step in this movement strategy.

In B.P.J., the court held that West Virginia and Idaho could ban trans athletes from participating in women’s and girls’ athletics because the states were simply recognizing physical differences between the sexes—what the court called biological sex. The court explained that Title IX was all about biological sex, with Justice Brett Kavanaugh in his majority opinion framing the case as a minor exception to the court’s sex-discrimination jurisprudence, one that reflects differences in size, speed, and strength that make sports unique.

Kavanaugh’s opinion is an example of reasoning from the body: The court discussed biological differences between the sexes as if they naturally justify laws that discriminate on the basis of sex. With this view, once a court recognizes biological sex difference, the court has no reason to scrutinize state action for sex-role stereotyping.

There are reasons to be suspicious of this account. Take the example of sports, the issue at the heart of B.P.J. Today Title IX, the 1972 civil rights law that prohibits sex discrimination in education programs receiving federal money, has transformed women’s and girls’ athletics. Following significant increases in the participation rates of female athletes in high school and........

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