Title VII of the Civil Rights Act of 1964 forbids employment discrimination based on “race, color, religion, sex, or national origin.” When the members of Congress wrote and passed it, they surely thought their meaning was straightforward. But now the meaning of “sex” has been contested all the way up to the Supreme Court.

In Harris Funeral Home, Inc. v. the Equal Employment Opportunity Commission, the justices must decide if Title VII now bans discrimination on the basis of gender identification. If so, that will impact not just family businesses like Harris, but everything from women’s shelters to single-sex athletic competitions. A girls’ track meet that refused to allow a trans runner to compete could wind up in the same legal hell as one that tried to ban black runners.

Harris Funeral Homes’ long march through the legal system began in 2013 when funeral director Anthony Stephens came back from vacation as Aimee Stephens. Thomas Rost and his wife, co-owners of the family business, say the surprise wouldn’t have been a problem — except that Stephens had agreed to follow a sex-specific dress code: suits for men, dresses and skirts for women, so that funeral directors offer a calm center for grieving families.

Asked to comply with the dress code, Stephens refused. And after long reflection, the Rosts fired Stephens.

Enter the Obama-era EEOC, which sued, insisting that “gender identity” is a facet of “sex” under the law. After several rounds of appeals, the last ruling holds that the EEOC was correct.

Will the Supreme Court agree? Most of the justices seemed unsure during oral argument, noting that Congress plainly didn’t intend that meaning of “sex” in 1964, and has never amended the law to adopt it. Yet some were open to the argument that the logic of anti-discrimination can and should now extend to categories no one even considered back in 1964.

One thing should be clear: Setting a reasonable standard here is more complex than most trans-rights advocates admit.

As the self-described radical feminists of the Women’s Liberation Front put it in their friend-of-the-court brief: “Legally redefining ‘female’ as anyone who claims to be female results in the erasure of female people as a class. If, as a matter of law, anyone can be a woman, then no one is a woman, and sex-based protections in the law have no meaning whatsoever . . . a ruling that Congress surely did not intend.”

Sorting out such issues requires more than re­defining “sex”; it’s a task the Supremes should leave to legislators.