None of the experts contacted by The Associated Press could recall another "no" vote in the 612-seat National Assembly, which meets briefly twice a year and approves laws by unanimous show of hands.
"This is the first time, without a doubt," said Carlos Alzugaray, a historian and former Cuban diplomat.
He said even measures that were widely criticized in grass-roots public meetings, such as a law raising the retirement age, had passed unanimously in the Assembly.
Few in Cuba were even aware of the Dec. 20 vote until after the measure was enacted into law this summer, at which point gay activists publicized the vote by Castro, who is the island's most prominent advocate for gay rights.
Arturo Lopez-Levy, a Cuban analyst who lectures at the University of Denver, suggested it might "open doors for other important initiatives."
Mariela Castro herself seemed to hint there could be more debate in the assembly.
"There have been advances in the way things are discussed, above all the way things are discussed at the grass-roots level, in workplaces, unions and party groupings," she said in an interview posted in late July on the blog of Francisco Rodriguez, a pro-government gay rights activist. "I think we still need to perfect the democratic participation of the representatives within the Assembly."
Others are skeptical it will set a precedent.
"I would say that this is more a sign of what Mariela can get away with than a sign of what your everyday parliamentarian can get away with," said Ted Henken, a professor of Latin American studies at Baruch College in New York.
In her crusade for gay rights, Castro has often taken stands that challenge the social status quo, while firmly supporting the Communist government.
The new labor code bans workplace discrimination based on gender, race and sexual orientation. But it has no mention of HIV status or gender identity.
"I could not vote in favor without the certainty that the labor rights of people with different gender identity would be explicitly recognized," Castro said in the blog interview.
Raul Castro himself has been slowly shaking up Cuba's system by allowing some limited private-sector activity and scrapping a much-loathed exit visa requirement. He's made it clear, though, that the Communist Party will continue to be the only one permitted.
The vast majority of Assembly members keep their regular jobs and are not professional lawmakers. Laws are generally drafted by a handful of legislators and discussed with Cubans before being presented to parliament.
There was no response to requests for an interview with Mariela Castro, who heads Cuba's National Center for Sex Education, an entity under the umbrella of the Health Ministry.
She has spoken in the past about wanting to legalize same-sex unions, though concrete legislation to that effect has not materialized.
That LGBT rights is even a matter of debate is a sign that much has changed since the 1960s and '70s, when gay islanders were routinely harassed and sent to labor camps along with others considered socially suspect.
In recent years, Fidel Castro expressed regret about past treatment of gays, and today Cuba's free and universal health care system covers gender reassignment surgery.
But activists say old attitudes and prejudices die hard so the LGBT community needs more legal protections.
Rodriguez and about 20 others from Project Rainbow, a group that advocates for sexual diversity, recently sent a public letter urging Mariela Castro to introduce legislation to amend the labor code.
"These are not minor details," Rodriguez said. "They are social problems we have in contemporary Cuba."
Associated Press writer Peter Orsi in Havana contributed to this report.
Andrea Rodriguez on Twitter: www.twitter.com/ARodriguezAP