During oral argument earlier this month in the case of a woman who was fired for being transgender, U.S. Supreme Court Justice Neil Gorsuch asked a question that I've been thinking about ever since.

Aimee Stephens was fired after she informed her boss of six years that she is a woman, though her sex assigned at birth was male. She argued her discharge was unlawful sex discrimination. (I am part of the legal team representing Ms. Stephens as well as the estate of Donald Zarda, who was fired for being gay and whose case was heard the same day as Ms. Stephens's).

Assume, Justice Gorsuch said, that the plain text of the law favors Ms. Stephens. Shouldn't a judge take into account that following the law to its natural conclusion would result in "massive social upheaval"? The implication was that recognizing equal opportunity for transgender workers would lead to turmoil.

The Court has consistently stated that its interpretations must be governed by the text of the law, not predictions about how society will respond. In any event, when it comes to transgender people in the workplace, such concerns are misplaced. Federal courts have protected transgender people from discrimination for 20 years, as my colleague David Cole, who argued the case on behalf of Ms. Stephens, rightly noted. Those rulings have caused no upheaval to individual workplaces, the American workplace at large or to society. For the Supreme Court to affirm that employers may not penalize workers for being transgender would not be sudden or disruptive.

What has disrupted social expectations and transformed the American workplace is the 55-year-old rule banning sex discrimination in the first place.

The notion that sex could—or should—be irrelevant to job opportunities was a radical one in 1964, when Congress passed Title VII of the Civil Rights Act, which bars workplace discrimination based on race, sex and other characteristics. At the time, newspapers routinely separated "help wanted" advertisements into jobs for women and jobs for men. The then-new Title VII ended the express division of jobs by sex.

Other changes were slower, but come they did, inexorably. In the "Mad Men" era of 1964 and continuing for close to two decades thereafter, judges wrote off sexual harassment as a "natural sex phenomenon" that workers were simply expected to tolerate. Again, it was Title VII—and the judges who interpreted it—that recognized such workplace harassment for what it was: conduct that imposed unequal "terms, conditions or privileges" of employment.

As recently as 1989, the Supreme Court heard the case of Ann Hopkins, who was told that in order to advance at her accounting firm, she needed to "walk more femininely, talk more femininely, dress more femininely, wear make-up, have her hair styled and wear jewelry." That such feedback on a job applicant's candidacy would be roundly condemned today is attributable in part to the Supreme Court's ruling in Ms. Hopkins's favor.

During the past 55 years, Title VII upended the social expectations that women could be precluded from certain jobs or required to behave in a traditionally feminine manner to get ahead. In other words, Title VII itself already caused "massive social upheaval." Confirming that the same norms apply to transgender workers would be a comparatively modest step for the law.

A clear sign that this cultural shift has already happened is the fact that Ann Hopkins's former firm, now known as PwC, joined a friend-of-the-court brief filed by more than 200 American businesses urging the Supreme Court to affirm application of Title VII to LGBTQ workers—in effect, opposing the rule it once asked the Court to adopt.

Though many employers are already on board, the stakes remain high for the 1.5 million transgender and 11.5 million lesbian, gay, and bisexual people whose livelihoods are on the line. Indeed, for many LGBTQ people, the ability to work is more fundamental than the freedom to marry.

According to a 2017 study by the Center for American Progress, one in four LGBTQ workers says they've experienced discrimination at work in the last year alone. That includes people like Elijah, who was fired from his job at a coffee shop after the owner learned he is transgender; he has been fired (or not hired) more than half a dozen times for the same reason. Or Laura, who served as a police officer for 16 years until she was the victim of a burglary that revealed her transgender status and was promptly fired. Or Yolanda, a lesbian forklift operator whose manager threatened her constantly with comments such as, "I want to turn you back into a woman." After Yolanda reported the harassment, she was fired.

The job market is even grimmer for LGBTQ people of color, who are at least twice as likely as white LGBTQ people to face discrimination when applying for jobs.

Title VII directed that we all should have the chance to succeed based on our own abilities, not our sex. Clarifying that the same is true for LGBTQ workers is simply the latest application of a law that long ago transformed our workplaces, and by extension, our very culture. Giving the green light to employers to fire people because of their sex, simply because they are LGBTQ, is the outcome that would cause the kind of "massive social upheaval" all of us should fear.

Ria Tabacco Mar is a senior staff attorney with the national ACLU's Lesbian Gay Bisexual Transgender & HIV Project, which represents Aimee Stephens and Donald Zarda in their cases before the Supreme Court. On Twitter: @RiaTabaccoMar