The transgender movement expects Americans to deny the most basic truths about bodily sex and human nature, and whether out of fear or apathy, most acquiesce. But 19-year-old Alexis Lightcap won’t go along. She insists that male and female bodies are different, and that the difference has important consequences for the privacy and dignity of men and women alike. She’s prepared to go all the way to the U.S. Supreme Court to make the point.

Lightcap is one of six student plaintiffs—the only one to go public—in a lawsuit challenging the transgender bathroom policy at Boyertown Area School District, about an hour outside Philadelphia, which allows students to use the bathrooms, lockers, and other private facilities of the opposite sex based solely on their own subjective sense of “gender identity.” Put another way: The school district permits biological boys to enter girls’ private facilities and vice versa.

One day in 2016, when she was a junior at Boyertown High, Lightcap found herself on the sharp end of the policy. “I was on my way to the girls’ bathroom,” she said in a phone interview Thursday, “when I suddenly saw a male inside washing his hands. I froze. Then I ran out in shock.” How did she know the student was male? “I’ve been going to this school since fourth grade”—it’s a tiny district—“I know everybody. Up to this moment, this person was a male.”

Lightcap immediately told a teacher about the incident. Concerned, her teacher urged her to alert administrators. But a principal told Lightcap and her teacher to “deal with it,” meaning the new policy. School officials told the other student-plaintiffs, who include boys and girls, that they need to “tolerate” changing with the opposite sex and to make it as “natural” as possible, according to a legal brief filed by the Alliance Defending Freedom (ADF), the religious-liberty law firm representing Lightcap and the other students. A spokeswoman for the school district declined to comment.

Boyertown Area School District, however, hadn’t bothered to inform the student body or parents about the policy change. Only the students who sought access to opposite-sex bathrooms were in-the-know. Not that a heads-up would have changed Lightcap’s mind.

“Guys don’t belong in girls’ bathrooms,” she told me, “and girls don’t belong in guys’ bathrooms. The school itself says it protects the privacy of every person. Students should not have to change in bathrooms where they don’t feel comfortable.” And although Lightcap graduated this week, the issue is still very much relevant to her: “My sister is still in that school. She’s in the seventh grade. So I don’t want males in her bathroom.”

The district hasn’t specified an age floor for the policy.

The ADF filed the lawsuit in March 2017 in the U.S. District Court for the Eastern District of Pennsylvania. Subsequently, it moved for a preliminary injunction to stop the policy from being implemented while the litigation made its way through the courts. The legal arguments on Lightcap’s side are compelling.

American law and tradition take it for granted that the sexes are entitled to safe, separate, and private facilities. Until relatively recently, progressives embraced the idea. A 1975 op-ed in the Washington Post, for example, argued that “separate places to disrobe, sleep, perform personal bodily functions are permitted, in some situations required, by regard for individual privacy. Individual privacy, a right of constitutional dimension, is appropriately harmonized with the equality principle.”

The author of that op-ed: Ruth Bader Ginsburg.

In numerous criminal law and correctional contexts, courts have found a Fourteenth Amendment fundamental right to privacy from the opposite sex viewing Americans’ nude bodies. Equally important, Title IX treats the use of private facilities by members of the opposite sex as sexual harassment, since it exposes people of one sex to the unwanted presence of the other sex. Title IX’s protections arose out of concerns about anatomical differences between the sexes; the notion of a “gender identity” separate from bodily sex would have confounded the law’s framers.

Indeed, the whole edifice of Title IX protections would collapse if the sex binary—objective, anatomical, tied to human biology—were to give way to the fluid and ever-shifting categories of gender identity, which flow from mental states. It would be a strange and bitter irony if a law enacted to protect the female sex in public settings were to be reinterpreted by the courts in such a way as to destroy biological sex as a legally meaningful concept.

Last August, the district court dismissed the ADF’s motion for a preliminary injunction, and an appeals hearing before the Third Circuit didn’t go much better. Third Circuit Judges Theodor McKee and Patty Schwartz railed against ADF lawyers for using the terms “biological sex” and “opposite sex.” Such language, McKee said, “complicates the discussion.”

He went on: “It is not clear at all. It does not—to my mind, that does not enhance the clarity. If I hear the term ‘transgender boy’ and ‘transgender girl,’ then I can get my head around that.” Since girls who identify as boys are treated the same as boys who identify as girls, the judge argued, there was nothing discriminatory about the district policy. He was begging the question, in other words. The whole debate revolves around whether Title IX sex protections mean anything once biological sex is abolished.

When the lawyer for Lightcap presented expert testimony on the tension between biological sex and gender identity, Judge McKee interrupted him: “Well, you’re in my house, or our house . . . So let’s use the term that to my mind is . . . most respectful to the persons involved here.” In other words, the appeals bench had adopted the transgender movement’s vocabulary and worldview from the outset.

Denial of a preliminary injunction, of course, doesn’t mean the case is dead. The litigation now returns to the district court for adjudication, and the plaintiffs are prepared for the long fight. Lightcap, especially, is unbowed. Having spent half her life bouncing between parents, grandparents, and the foster-care system, she has a powerful sense of mission and destiny. “This was God’s plan for me,” she told me. “If I didn’t follow what He wanted me to do, I don’t know what I would do.”