Should the legal meaning of “sex” be changed? That’s what the Supreme Court will consider Tuesday when it hears oral arguments in the cases of Harris Funeral Homes v. Equal Employment Opportunity Commission and Bostock v. Clayton County. Should the justices decide to broaden the meaning of sex, it would spell disaster for women.

The cases relate to whether federal law prohibits employment discrimination based on employees’ sexual orientation (Bostock) or “presenting” as the opposite sex (Harris Funeral Homes). Many Americans...