In Idaho, a 1921 law remains on the books holding that unmarried people who have sex “shall be found guilty of fornication.” The statute was essentially defunct until 1996, when Gem County, Idaho, resurrected it as a method to discourage teenage pregnancy. In applying the law, Gem County prosecutor Douglas Varie targets pregnant teenagers and their boyfriends. One such teenager, seventeen-year-old Amanda Smisek, was convicted and sentenced to three years of probation, plus parenting classes.

Among Varie’s critics is the American Civil Liberties Union (ACLU). Jack Van Valkenburgh, executive director of the ACLU of Idaho, states, “To the extent that the prosecutions are targeting teenagers, and the law applies to everybody, it is selective prosecution, and it denies equal protection of the law.” Moreover, warn opponents, while the law will not keep teenagers from having sex, the...