The following is excerpted from McGuire's new book, "Sex Scandal: The Drive to Abolish Male and Female"

Feminists in the 1960s and 70s argued that men and women are not inherently different. The many apparent differences between the sexes — beyond the undeniable anatomical ones — are simply the result of gender roles people are taught to fulfill, not of their natures as men or women. This was the era when parents were told that their daughters would be just as happy playing with toy trucks as with dolls and that making the switch would help end sexism and liberate girls for a better future. Women sought to detach themselves from the aspects of womanhood they found limiting, especially their fertility.

For decades, gender theory gained steam, seeking the complete abolition of gender distinctions in any way tied to the two biological sexes.

In 1992, family therapist John Gray published Men Are from Mars, Women Are from Venus. The book's premise was simple: men and women are different, and understanding those differences, not living in denial of them, is the key to relationship success. The book's popularity exploded. It became not just a bestselling book of its decade, but one of the bestselling books of all time.

It was pop psychology, but it hit a social nerve. The book hit shelves amid a growing effort to build a gender-neutral society. It seemed people were still desperate to understand their differences. Paradoxically, the dawn of gender theory was also the period when social scientists and doctors began to make the most progress in understanding the sexual difference and the delicate physiological interplay between men and women.

Major advances in neurobiology, for example, unveiled just how differently men and women respond physically to intimacy. During intercourse, the female releases more oxytocin than the male. Oxytocin is the hormone that facilitates bonding between human beings, in particular between mothers and new babies and between heterosexual partners. It's colloquially referred to as "the love hormone," "the hug hormone," the "cuddle chemical," the "moral molecule," and the "bliss hormone," and is especially noted for the different roles it plays in female reproduction.

According to the American Psychological Association, "New studies are adding to a body of literature that shows oxytocin plays a key role in maternal bonding and social affiliation — what [social psychologist Shelley] Taylor has labeled the 'tend and befriend' response, as opposed to the 'fight or flight' response."

Oxytocin, researchers discovered, makes a woman more vulnerable and attached to the man with whom she is having sex. Men release a small amount of oxytocin during intercourse, too, but they release an even bigger amount of testosterone, which has the effect of suppressing the oxytocin.

So science has a basic explanation for why women will stare at their phone after casual sex, hoping their partner will contact them, while men do not. As one woman wrote in a piece for Elite Daily, "The Truth Behind Why Women Find It Harder to Have Casual Sex than Men Do," the phenomenon of oxytocin offers "a scientific explanation as to why after sex, women are left wondering if and when she will hear from a guy. All the while, guys are scrolling through Tinder on their couch, wondering if that chicken parm they ordered an hour ago is actually on its way." "Women," she writes, "are programmed to become emotionally attached" in a way that men are not.

There is a seemingly unending litany of ways that men and women are different, many of them still unexplained.

As Harvard professor Harvey Mansfield notes in Manliness, his treatise on manhood, data show that women are friendlier. Two-thirds of people who are more inclined toward smiling are women.

Women are also more communicative. Multiple studies have found that women, on average, say more words than men in a given day, with one estimate putting the difference at approximately 20,000 to 7,000. The difference in verbal facility between males and females is a phenomenon that begins early, as any parent who's had children of both sexes can attest to.

That difference persists into adulthood, and scientists now ascribe it in part to higher levels of the protein FOXP2 in the female brain. As one researcher who studies sex differences at Michigan State put it, "Higher levels of FOXP2 expression are found in the more communicative sex in each species." According to another scientist at the Society for Neuroscience, the FOXP2 link "raise[s] the possibility that sex differences in brain and behavior are more pervasive and established earlier than previously appreciated."

On the other hand, there was the study released in 2015, based on MRI images, finding no evidence that there is such a thing as a "male" or "female" brain. In the words of one science journal, "The majority of the brains were a mosaic of male and female structures." As was to be expected, gender theorists promptly declared victory in the debate about sexual difference. "There is no such thing as the 'male brain' or 'female brain,' and scientists have the scans to prove it," exclaimed the Los Angeles Times, and about one million other news outlets.

But as Louann Brizendine, an Ivy-educated neuropsychiatrist and author of The Male Brain and The Female Brain, summarized the science of sex differences in a piece for CNN.com, "Our brains are mostly alike. We are the same species, after all. But the differences can sometimes make it seem like we are worlds apart." Or to some, planets apart.

Medical phenomena continue to point to stark differences between the male and female brains.

Men, for instance, are significantly more likely to have reading disorders, something that has been attributed to, among other things, "differences in brain functioning." And reading disorders may arise differently in men and women. According to a Georgetown University Medical Center study, "Brain anatomy of dyslexia is not the same in men and women, boys and girls."

The study's lead author explained, "There is sex-specific variance in brain anatomy and females tend to use both hemispheres for language tasks, while males just the left. It is also known that sex hormones are related to brain anatomy and that female sex hormones such as estrogen can be protective after brain injury, suggesting another avenue that might lead to the sex-specific findings reported in this study."

The world of science and medicine is trending toward greater, not lesser, understanding of the importance of what makes us different.

Does great variety exist within the sexes? Of course. Some men are poets, some men are soldiers. Some women are trial lawyers, while others write books. Some men vacuum. Some women don't cook.

And yet, despite historical changes in fashion preferences, domestic arrangements, professional inclinations, and so much more, certain aspects of nature stubbornly persist. Shifts in who wears pink have not changed the fact that women, and only women, can conceive, gestate, and give birth to a new member of the human species. Men are, on the whole, the physically stronger sex. Culture may imbue gender with certain random characteristics, but science is not walking away from sex as a key feature of humanity.

Culture may change, but reality doesn't.

These days we hear about "climate change deniers." But the full-blown denial of something much more obvious—a reality that both science and our lived experience make plain—which started in the sixties has now reached critical mass. At their best, sex deniers sought to understand the complexities within the sexes. But they have completely overshot and now deny the very real differences that define us.

Ashley McGuire is author of Sex Scandal: The Drive to Abolish Male and Female. If you would like to write an op-ed for the Washington Examiner, please read our guidelines on submissions here.