The Senate spent the better part of three months in 1964 debating, amending and voting on the Civil Rights Act. Not once did anyone in the chamber use the phrase “sexual orientation.”

Yet 55 years later, the Supreme Court has been asked to rule that the law does, in fact, cover sexual orientation in its Title VII ban on employment discrimination based on sex. The court has been asked to add “gender identity” as a protected class.

The three cases dealing with the Civil Rights Act are some of the biggest facing the justices in a term that is already shaping up to be momentous. Major decisions are expected on immigration, presidential powers and gun rights.

“It’s going to affect so many millions of lives,” said Gregory Antollino, the lawyer representing the estate of Donald Zarda, who was fired from his job as a skydiving instructor in 2010 after he revealed his homosexuality to a customer.

The customer later reported the interaction to Zarda’s boss. Zarda said he was fired because of the incident, though the company, Altitude Express, denies the allegation.

Zarda died in a skydiving mishap in 2014, but his sister is fighting the case on behalf of his estate against his former employer.

Zarda won at the 2nd U.S. Circuit Court of Appeals, which overturned its own precedent and decided that the Civil Rights Act did cover discrimination based on sexual orientation.

Altitude Express appealed. It told the Supreme Court that the 2nd Circuit broke not only with its own previous rulings but also with decades of precedent in other appeals courts. The company said it’s up to Congress to rewrite the law.

The Trump administration agreed.

In briefs filed last month, the Justice Department argued that Congress was clear on what it intended in discrimination law and that didn’t include sexual orientation. Solicitor General Noel Francisco said Congress can, of course, rewrite the law to expand the categories of discrimination.

Congress has considered a number of bills to do just that, and all of them have failed.

Mr. Francisco said businesses can offer their own guarantees but the court would be overstepping if it created the right on its own.

Gay rights groups counter that the Supreme Court expanded its understanding of sex in a 1989 case involving Ann Hopkins, who said she had been denied a promotion because she wasn’t feminine enough and was told to wear makeup and act less aggressively.

The high court at the time said that amounted to sex discrimination because she was punished for flouting a female stereotype.

Zarda, in his case, said sexual orientation is another way of viewing stereotypes — the expectation that a man would be attracted to a woman rather than another man.

“Just as discrimination against Ann Hopkins based on the stereotype that women should be feminine and deferential was discrimination ‘because of sex,’ discrimination against Zarda because of the stereotype that men should be attracted only to women is discrimination ‘because of sex,’” his attorneys argued to the justices.

On Oct. 8, the same day the Zarda case is scheduled, the justices are also scheduled to hear arguments in a dispute involving Gerald Bostock, who says Clayton County, Georgia, fired him from his job as a child welfare service coordinator after the court system learned he was homosexual and played in a gay sports league, Hotlanta Softball.

The court system says he was fired because he mismanaged funds.

The 11th U.S. Circuit Court of Appeals dismissed Mr. Bostock’s case, saying his discrimination claim couldn’t stand because the Civil Rights Act doesn’t cover sexual orientation.

The third case in the justices’ combined docket deals with gender identity, which may not match a person’s sex.

Thomas Rost, a devout Christian who owns a funeral home, fired a funeral director who maintained a male gender identity for six years at work and then started presenting as a woman. Mr. Rost argued that was a problem for his business, which had a long-standing dress code for men and a different dress code for women. He said a reversal could be jarring for grieving families and that a funeral director’s chief role is to blend into the background.

The 6th U.S. Circuit Court of Appeals disagreed, saying “sex” as defined in the Civil Rights Act has come to mean gender identity.

John Bursch, senior counsel for the religious liberty firm Alliance Defending Freedom, which is defending the funeral home, said the law is clear and that people should be able to rely on language that Congress wrote.

“Title VII and other civil rights laws, like Title IX, are in place to protect equal opportunities for women; changing ‘sex’ to mean ‘gender identity’ undermines that,” he told The Washington Times.

About half the states have laws that extend employment discrimination protections to sexual orientation or gender identity, but activists want a federal backstop to protect LGBTQ individuals in the other states.

“It’s not just again the decision which is something that we understand to be harmful — the decision to fire you — but it also includes harassment, so it’s to say sexual orientation is not included really puts certain people out of protection,” said Tristin Green, a law professor at the University of San Francisco.