The justices on the U.S. Supreme Court are getting a lesson in "sex."

"'Sex' is an objective biological reality, and law based on this objective reality has a solid footing," explains a new friend-of-the-court brief filed against a school district that instructed students to act as "natural" as possible when they shower with someone of the opposite sex.

"In contrast, 'gender identity' is an amorphous subjective 'inner sense.' Basing law on such a subjective sense leads not to clarity and certainty, but to incoherence and confusion," the brief argues.

The brief is by Ryan Anderson, Ph.D., the author of "When Harry Became Sally: Responding to the Transgender Moment" as well as "Truth Overruled: The Future of Marriage and Religious Freedom."

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It was submitted in a case brought by students against the Boyertown Area School District in Pennsylvania for allowing students to use rest rooms and showers according to "gender identity" rather than biological sex.

Simply, a boy who says he's a girl can use the girls' showers.

Lawyers for the Alliance Defending Freedom are representing Boyertown students and their parents in the case.

Anderson's comments were submitted by the National Center for Law and Policy.

The legal team explained: "The litigation is the result of the Obama administration’s unilateral and extraordinary decision in 2016 to issue a written edict to public schools which, by fiat, interpreted Title IX's sex discrimination provisions ('no person in the United States shall, on the basis of sex, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any education program or activity') to require public schools to allow students to access bathrooms and locker rooms based on 'an individual's internal sense of gender.'

"As a result, transgenderism, also known as gender dysphoria, was read into a statute written solely to address sex, not gender identity. As a result, bathroom and locker room access in many public schools, including in the Boyertown Area School District, became based on subjective feelings, not objective facts or biology," the team explained.

"Sex has an objective meaning that corresponds to reality," said Dean Broyles, president of the center.

"Science, including biology and genetics, affirms there are only two genders, male and female. While we need to show compassion and understanding to students dealing with gender dysphoria, there are much better ways to accommodate them than forcing all students into intimate spaces with students of the opposite sex. The petitioning students' dignity and privacy deserves be acknowledged, tolerated and respected – not derisively ignored and dismissed as the lower courts unfortunately did."

He said: "Fortunately, courts have affirmed that males and females have a constitutionally protected privacy interest in their partially clothed bodies – acknowledging that forcing boys and girls to share locker rooms or restrooms with students who are objectively of the opposite sex can cause embarrassment and distress. Quite ironically, the school district is itself violating Title IX because its policy effectively denies students access to privacy facilities based on their sex (gender). Most children who experience gender dysphoria grow out of it. Schools can acknowledge the dignity and worth of students experiencing feelings of transgenderism without trampling on the privacy rights of all other students. It is a very slippery slope indeed if we start basing important public policy and legal decisions on the subjective transitory feelings of children."

The brief explains Congress and the government know the difference, because "gender identity" has been listed with "sex" in existing law, such as the Violence Against Women Reauthorization Act of 2013 and others.

Obama also knew the difference.

"In his executive order barring federal contractors from 'discrimination' on the basis of 'sexual orientation and gender identity,' he replaced existing protections on the basis of 'sex' with protections of the basis of 'sex, sexual orientation, gender identity,'" showing he did not consider gender identity protections to be legally included in the protections on the basis of sex," the brief said.

Scientifically it's simple, he explained. Females are XX chromosomes and males XY.

And that is unchangeable.

"This really isn't that controversial. Sex is understood this way across species. No one finds it particularly difficult – let alone controversial – to identify male and female members of the bovine species or the canine species. Farmers and breeders rely on this easy distinction for their livelihoods. It's only recently, and only in the human species, that the very concept of sex has become convoluted, and controversial," the brief said.

ADF earlier asked the Supreme Court for review after the 3rd U.S. Circuit Court of Appeals agreed with the district that it should have a take-your-pick policy on showers.

During the 2016-17 school year – without informing parents or students – the Boyertown Area School District opened its high-school locker rooms and restrooms to students of the opposite sex based on the students' beliefs about their gender, said the legal group.

"Some male students learned of the policy while they were undressing in their locker room and discovered that a female student was changing clothes with them," ADF said. "Embarrassed and confused, the students sought help from school officials, who told them they should just 'tolerate it' and 'make it as natural as possible.'"

ADF Senior Counsel and Vice President of Appellate Advocacy John Bursch said everyone should be able to agree "that students struggling with their beliefs about gender need compassionate support."

"But there are sound reasons why schools have always separated male and female teenagers in showers, restrooms, and locker rooms," he said. "No student's recognized right to bodily privacy should be made contingent on what other students believe about their own gender."

ADF contends the school violated the students' fundamental right to bodily privacy under the U.S. Constitution.

"These types of school policies have serious privacy implications," said ADF Legal Counsel Christiana Holcomb. "Teenagers should not be forced to welcome members of the opposite sex into their showers, restrooms, or locker rooms based on what they believe about their gender. All schools, including Boyertown Area School District, should be providing support for those dealing with gender dysphoria, but they should seek to do so in ways that protect the privacy of all students.

"It is untenable that the Third Circuit made students' right to bodily privacy contingent on what others believe about their own gender," the filing states. "Recognizing this reality does not diminish the concern for students who believe they are of the opposite sex. Schools can (and should) teach that every student has inherent dignity and worth and should be treated as such. Schools can (and should) assure students with gender dysphoria that they are valuable and important members of the school community. And school officials can (and should) provide them with resources and support. Despite such alternatives, Boyertown chose to violate the privacy rights of all other students."