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Transgender ideology can take on a comical character, as in a recent American Civil Liberties Union commentary objecting to sales tax on tampons and similar products while pondering: “How can we recognize that barriers to menstrual access are a form of sex discrimination without erasing the lived experiences of trans men and non-binary people who menstruate, as well as women who don’t?”

Yet it’s one thing to claim that a man can “identify” as a woman or vice versa. Increasingly we see a dangerous and antiscientific trend toward the outright denial of biological sex.

“The idea of two sexes is simplistic,” an article in the scientific journal Nature declared in 2015. “Biologists now think there is a wider spectrum than that.” A 2018 Scientific American piece asserted that “biologists now think there is a larger spectrum than just binary female and male.” And an October 2018 New York Times headline promised to explain “Why Sex Is Not Binary.”

The argument is that because some people are intersex—they have developmental conditions resulting in ambiguous sex characteristics—the categories male and female exist on a “spectrum,” and are therefore no more than “social constructs.” If male and female are merely arbitrary groupings, it follows that everyone, regardless of genetics or anatomy should be free to choose to identify as male or female, or to reject sex entirely in favor of a new bespoke “gender identity.”

To characterize this line of reasoning as having no basis in reality would be an egregious understatement. It is false at every conceivable scale of resolution.

In humans, as in most animals or plants, an organism’s biological sex corresponds to one of two distinct types of reproductive anatomy that develop for the production of small or large sex cells—sperm and eggs, respectively—and associated biological functions in sexual reproduction. In humans, reproductive anatomy is unambiguously male or female at birth more than 99.98% of the time. The evolutionary function of these two anatomies is to aid in reproduction via the fusion of sperm and ova. No third type of sex cell exists in humans, and therefore there is no sex “spectrum” or additional sexes beyond male and female. Sex is binary.

There is a difference, however, between the statements that there are only two sexes (true) and that everyone can be neatly categorized as either male or female (false). The existence of only two sexes does not mean sex is never ambiguous. But intersex individuals are extremely rare, and they are neither a third sex nor proof that sex is a “spectrum” or a “social construct.” Not everyone needs to be discretely assignable to one or the other sex in order for biological sex to be functionally binary. To assume otherwise—to confuse secondary sexual traits with biological sex itself—is a category error.

Denying the reality of biological sex and supplanting it with subjective “gender identity” is not merely an eccentric academic theory. It raises serious human-rights concerns for vulnerable groups including women, homosexuals and children.

Women have fought hard for sex-based legal protections. Female-only spaces are necessary due to the pervasive threat of male violence and sexual assault. Separate sporting categories are also necessary to ensure that women and girls don’t have to face competitors who have acquired the irreversible performance-enhancing effects conferred by male puberty. The different reproductive roles of males and females require laws to safeguard women from discrimination in the workplace and elsewhere. The falsehood that sex is rooted in subjective identity instead of objective biology renders all these sex-based rights impossible to enforce.

The denial of biological sex also erases homosexuality, as same-sex attraction is meaningless without the distinction between the sexes. Many activists now define homosexuality as attraction to the “same gender identity” rather than the same sex. This view is at odds with the scientific understanding of human sexuality. Lesbians have been denounced as “bigots” for expressing a reluctance to date men who identify as women. The successful normalization of homosexuality could be undermined by miring it in an untenable ideology.

Those most vulnerable to sex denialism are children. When they’re taught that sex is grounded in identity instead of biology, sex categories can easily become conflated with regressive stereotypes of masculinity and femininity. Masculine girls and feminine boys may become confused about their own sex. The dramatic rise of “gender dysphoric” adolescents—especially young girls—in clinics likely reflects this new cultural confusion.

The large majority of gender-dysphoric youths eventually outgrow their feelings of dysphoria during puberty, and many end up identifying as homosexual adults. “Affirmation” therapies, which insist a child’s cross-sex identity should never be questioned, and puberty-blocking drugs, advertised as a way for children to “buy time” to sort out their identities, may only solidify feelings of dysphoria, setting them on a pathway to more invasive medical interventions and permanent infertility. This pathologizing of sex-atypical behavior is extremely worrying and regressive. It is similar to gay “conversion” therapy, except that it’s now bodies instead of minds that are being converted to bring children into “proper” alignment with themselves.

The time for politeness on this issue has passed. Biologists and medical professionals need to stand up for the empirical reality of biological sex. When authoritative scientific institutions ignore or deny empirical fact in the name of social accommodation, it is an egregious betrayal to the scientific community they represent. It undermines public trust in science, and it is dangerously harmful to those most vulnerable.

Mr. Wright is an evolutionary biologist at Penn State. Ms. Hilton is a developmental biologist at the University of Manchester.