Glastonbury, Conn.

As a star high-school athlete, Selina Soule doesn’t shrink from the spotlight—but she never planned to gain it in the manner she has. This year the 16-year-old has appeared on Fox News more than once to express her opposition to the transgender policy of her state’s athletic conference. Since 2017, Connecticut schools have allowed young men to displace Ms. Soule and other girls in sports competitions. Across the country, controversies around women’s sports have become one of the sharpest fault lines in the national debate about transgender issues.

Last month Alliance Defending Freedom filed a civil-rights complaint with the Education Department on behalf of Ms. Soule and two other Connecticut girls. They argue that allowing boys to compete in the female category denies girls “opportunities for participation, recruitment, and scholarships,” contravening Title IX’s prohibition of discrimination on the basis of sex.

Presumably, the legislators who passed Title IX in 1972 understood sex to be anatomical. But today its text poses an unforeseen challenge to administrators. They must decide whether the definition of sex includes “gender identity”—one’s sense of being male, female or neither. Connecticut’s is one of 19 state athletic conferences that allow athletes to compete based solely on their expressed gender identity. In contrast, both the International Olympic Committee and the National Collegiate Athletic Association require male-to-female transgender athletes to take testosterone-suppressing drugs to compete in the women’s category.

The results speak for themselves. Since Connecticut’s athletic conference enacted its liberal gender-identity policy, two men have won 15 women’s state championships—titles that were held by 10 different Connecticut girls the previous year.