When it comes to something as straightforward as gender, many progressives are confused.

New York City recognizes 31 genders — thanks in large part to social justice warrior Mayor Bill de Blasio. The list includes “drag king,” “drag queen,” “butch,” “femme queen,” “gender fluid,” “gender blender,” “gender gifted,” “gender bender,” and “femme person of transgender experience,” among many others. And that’s just one example in one city.

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It turns out that long list is still not comprehensive enough. Now there are “they-bies,” kids whose parents keep their gender a secret from them and prefer to raise them free of any “gender stereotypes” — and let them “figure it out” for themselves.

Nate and Julia Sharpe of Cambridge, Massachusetts, have decided not to reveal the biological sex of their three-year-old twins to anyone — including the toddlers themselves — as NBC recently reported.

Instead, this couple prefers to explore gender-free parenting, an aim, they say, to shield their children from “harmful” gender roles that hard-wire them to be feminine or male.

“Actually, ‘gender stereotypes’ don’t hard-wire. Biology hard-wires,” Laurie Higgins, a writer for the Illinois Family Institute (IFI) in Tinley Park, Illinois, told LifeZette. “‘Gender stereotypes are what soft-wire — and these parents are engaged in soft-wiring. They are culturally conditioning their child to believe that objective, immutable biological sex is essentially meaningless.”

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The Sharpes even objected to being asked the question of whether they had boys or girls.

“We definitely got more pushback from co-workers, who were, like: ‘Wait, you’re not going to tell me what you’re having? You’re not going to tell me what your kids are?'” Julia Sharpe told NBC. “I’m like, ‘I’m telling you they’re children.’ But they got really, really frustrated that we wouldn’t tell them what their genitalia was, which is kind of a weird thing, when you think about it.”

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No. Naming the sex of a baby — boy or girl — is not weird at all. It’s straightforward.

Higgins sees through the insulting “explanation” by Sharpe. “Claiming that asking about the sex of a baby is asking about ‘their genitalia’ is a disingenuous way of framing such an interaction,” she said. “Yes, genitalia is the obvious signifier of a baby’s objective, immutable biological sex, which is what friends and family are inquiring about. They’re not inquiring about genitalia per se … which is what Julia Sharpe was trying to imply. They’re just inquiring about a baby’s sex.”

Listen to the segment on this from “The Laura Ingraham Show” on Wednesday:

The “theyby” trend is also gaining traction among millennials. Theybies are encouraged to dress the way they feel and play with gender-neutral toys — and are referred to with such gender-neutral pronouns as “they,” “them,” and “their.”

What they fail to realize, however, is that hard science strongly supports male and female differences.

Scientists recently found 6,500 genes that are expressed differently in males and females, which helps explain the different likelihoods of certain diseases, or how drugs are absorbed differently.

Shmuel Pietrokovski and Moran Gershoni of the Weizmann Institute of Science in Israel developed a comprehensive map of genes that are differentially expressed in men and women, as a piece in The Scientist noted.

Still, some people can’t see the writing on the wall.

In the nonbinary world, everything is subjective — which could eventually set up kids for even more failure, especially where gender-free parenting is not the norm.

“There’s a problem of sex and gender inclusion at all levels of science, from faculty to the animals we use,” Rebecca Calisi, assistant professor of neurobiology, physiology and behavior at the University of California, Davis, told News-Medical.net. “We’re trying to ameliorate that, at least in reproductive biology.”

In the meantime, confusion and vague platitudes abound. “A theyby is, I think, different things to different people,” said Nate Sharpe. “For us, it means raising our kids with gender-neutral pronouns — so, ‘they,’ ‘them,’ ‘their,’ rather than assigning ‘he,’ ‘she,’ ‘him,’ ‘her’ from birth based on their anatomy.”

That’s the problem. In that nonbinary world, everything is subjective — which could eventually set up kids for even more failure, especially where gender-free parenting is not the norm.

Related: Science Guy Bill Nye ‘Changes’ the Facts on Gender

“Once your child meets the outer world, which may be day care, or preschool, or grandparents — it’s pretty much impossible to maintain a gender-free state,” Lise Eliot, professor of neuroscience at the Chicago Medical School and author of “Pink Brain, Blue Brain,” told NBC. “And depending on how conventional your community is, you could be setting your child up for bullying or exclusion.”

Elizabeth Economou is a former CNBC staff writer and adjunct professor. Follow her on Twitter.