Why are young adults turning out on the streets more, raging against unwelcome guest speakers on their campuses more? Why are they parading Nazi emblems or smashing statues of yesterday’s heroes? What are they not doing that gives them so much time and energy to burn?

Sex. In America, anyway, they are not doing sex. According to an expert on the subject, sociologist Mark Regnerus, “people born in the 1930s had the most sex, whereas those born in the 1990s are reporting the least.” That’s what research published recently in the journal Archives of Sexual Behavior reveals.

And this despite the fact that, as Regnerus argues in a new book, sex has become so cheap. “Fifty years on from the advent of the sexual revolution, we are witnessing the demise of eros,” he writes in an article in the October issue of First Things.

While this revelation will shock some people, it was only to be expected. Even today, he continues, “most sex happens within long-term, well-defined relationships.”  But look what has happened to marriage: in the few short years between 2000 and 2014 the proportion of Americans who were married declined from 55 percent to 41 percent. Young Americans are now more likely to express passion for Black Lives Matter or saving the planet than for another person.

Even younger Americans who are married may find more excitement in the “ping of an incoming text message or new Facebook post” than in intimacy with their spouse. Pornography provides another substitute. And some of the under-40s are, no doubt, among the 1 in 8 adults taking anti-depressant medication, which commonly decreases libido.

Behind the pills and the pings and the porn, however, Regnerus finds something much bigger going on: the deadening effect on eros of the idea that men and women must be equal in every domain. The data is in, he says: “Equality between the sexes is leading to the demise of sex.”

Of course, this is not what the gender equality industrial complex wants to hear. In all likelihood they will rubbish this conclusion, as they have Regnerus’ findings about same-sex parenting, but it makes sense. In summary, the story goes like this.

With the help of the Pill (and abortion when it failed) women increased their presence in the workforce and their earnings relative to men. As a result they had less reason to marry, as did men, who could get sex cheaply – that is, without the costs in fidelity, time and finances of marriage. Women became more equal to men (and indeed more like them in sexual habits) but there was a hitch. Men could afford to become picky about a spouse and bide their time over marriage; women, who are certainly not like men when it comes to fertility, now find themselves – often after the misery of abortions and several failed relationships — having to settle for Mr Not Quite Right, or no-one.

Furthermore, when they do get married, they are expected go for a 50-50 partnership all the way down the line: income, housework, childcare and so on. The fact that some accomplished women don’t even want this kind of equality caused sociologist, Leslie Bell, to lament that their “unprecedented sexual, educational, and professional freedoms” have led to “contradictory and paradoxical consequences.” Regnerus comments:

“Nonsense, I say. The only contradictory and paradoxical thing here is the unrealistic expectation of so many that the financial independence of women would have wholly positive effects on the dance of the sexes. Women and men still want each other, but the old necessities that once brought them together have disappeared.”

Sociologists who hoped that gender equality in marriage would lead to more sex were disappointed when they recently analysed the National Survey of Families and Households. It showed that husbands who do little or no housework had sex with their wives nearly two more times per month than did husbands who do all of it; and husbands who mow the lawn and fix things also have more sex.

“Men and women are not attracted to sameness, but to difference. We long for what is missing in ourselves. Needing each other makes us want each other.”

Egalitarianism, with its cheap sex and delayed marriage, has only made men and women lonelier and less connected than they once were, Regnerus concludes. “We cannot have both eros and strict equality between the sexes. Saving one requires sacrificing the other.”

As I suggested earlier, sexual egalitarians will be outraged by Dr Regnerus’ essay. They would rather cling to their bargain basement idea of sex and their soviet-style theory of marital equality than acknowledge what bad deal it has been for women – and for men. But read the whole article and you will see the truth of it all around you.