“Men need to feel a cold spike of fear when they begin a sexual encounter.” Those were not the words of a nun at your local Catholic high school 50 years ago. They were written in 2014 by the progressive Ezra Klein, editor of Vox, in explaining the importance of California’s “affirmative consent” law.

Passed in 2015, the law requires that students at the state’s colleges must obtain “affirmative, conscious and voluntary agreement to engage in sexual activity” from their partners. If not, they could be subject to disciplinary procedures for sexual assault.

It is one of the astounding ironies of our current era that universities, which have long been billed as havens of sex, drugs and rock ’n’ roll, places to experiment and rebel before you grow up, now employ armies of bureaucrats to regulate the sex lives of their students. These administrators hand out condoms and invite students to lectures by professional dominatrixes, while at the same time holding secret tribunals to punish men who engage in what can best be described as regrettable drunken hookups with their female classmates.

How did we get here? In their new book “The Campus Rape Frenzy,” KC Johnson and Stuart Taylor offer a detailed history of the panic over sexual assaults on campus and the use and abuse of Title IX to prosecute the alleged offenders.

The earliest foundations for the notion that rape is widespread on campus — indeed, according to statistics provided by activists, women are in more danger at Harvard than inner-city Detroit or Syria, for that matter — came from feminist scholars in the 1970s. Catharine MacKinnon famously wrote that “the similarities between . . . rape (and battery) on the one hand and intercourse on the other . . . makes it difficult to sustain the customary distinctions between violence and sex.” For a particular generation of feminism, all sex was basically rape.

As the barriers between men and women on campus came tumbling down and students were free to get inebriated and engage in casual sex, the number of regrettable encounters began to rise. Activists were not just interested in taking back the night from strangers who attacked women walking across campus. They wanted to give women the upper hand at all times.

Title IX of the Civil Rights Act offered them this opportunity. The provision states that “no person in the Untied States shall, on the basis of sex, be excluded from participation in, be denied the benefits of, or be subjected to discrimination under any education program or activity receiving federal financial assistance.” While Taylor and Johnson note that “at no point during the 1972 debate [over Title IX] did any member of Congress suggest that Title IX would apply to college disciplinary issues of any kind,” the provision is now the single most important factor driving college policy on sexual assault.

During the 1990s, the Office of Civil Rights decided that Title IX could not only protect women against harassment or discrimination by university employees but that it would also apply to relationships with other students. And universities could be held liable for a “hostile environment.” But things got worse under the Obama administration, which in 2011 issued a “Dear Colleague” letter that detailed certain disciplinary processes universities must use to adjudicate sexual assault complaints. These processes lowered the standard for finding a student guilty. They encouraged colleges to keep students who hadn’t been found guilty away from their accusers. They made it harder for accusers to defend themselves by banning direct cross-examination of accusers and they allowed accusers to appeal not-guilty decisions.

The authors show how the lives of many young men have been ruined by [rape] accusations.

These policies are premised on the notions that false accusations of rape are exceedingly rare (something that Johnson and Taylor go a long way to disproving) and that law enforcement is an insufficient tool to catch real offenders (though why physics professors are better at sorting out the details of a rape case than the police remains a mystery).

The Duke lacrosse case that occurred 10 years ago may have been an outlier in the kind of fantastical claims made by the victim, but it was hardly unusual. The authors present dozens of cases in which students clearly had consensual (if often drunken) sex, sometimes on multiple occasions, before the women involved found themselves encouraged by friends or administrators to call it rape.

The authors also show how the lives of many young men have been ruined by such accusations. They are expelled from school, find it hard to be admitted to another school. And thanks to the internet, these accusations follow them to job interviews and into their social lives.

But neither the administrators nor the media nor the public seem to have learned much from Duke or the many other cases in which the accusers have been exonerated. Which is how a Rolling Stone reporter managed to get an entire fraternity suspended at the University of Virginia and an entire country up in arms over a rape that was simply made up in order to make another boy jealous.

Students at UVA were demanding action before even the basic facts of the case were confirmed. Which is a real reason men on college campuses should feel “a cold spike of fear.”