“Sex!” exclaimed Gregory Peck, in the role of an Egyptologist striving to awaken a torpid Oxford lecture room. He was co-starring with Sophia Loren in Arabesque, Stanley Donen’s comedy thriller, in 1966. I would not recommend the same strategy even to a fictional lecturer now.

Prudery has replaced prurience. Some students at my university, who usually abhor censorship, recently demanded a “pornography filter” to exclude what they regard as unduly erotic websites from campus computers. I recoil from the idea, partly because it is impractical, partly because it is illiberal, but mainly because I have a certain feeling of gratitude for pornography. I fell in love at a debate about it in 1974. My fellow guest speaker, on the opposing side, was Lord Longford – then at the height of his anti-erotica clean-up “crusade”. Having just attended Haile Selassie’s obsequies, he arrived in morning dress, bedecked with decorations, which my future wife, who was secretary of the debating society, was obliged to help him doff. I lost the debate but won the woman. Ever after, Longford claimed that, thanks to him, a virtuous girl had redeemed a morally raddled defender of sin.

Everyone at the time predicted that sex, in Cole Porter’s words, was “here to stay” and that liberalisation of attitudes to display and practice was irreversible. The renaissance of disapproval has been among the surprising historical developments of my lifetime. Paradox has riven the process. Larry Flint could no longer pose as a liberal hero, but on the upper decks of buses, matrons of impeccable respectability amuse themselves with Fifty Shades of Grey. An advocate of women’s rights over their own bodies should approve or admit prostitution, but the bien-pensants are driving sex workers back into an underworld run by criminal pimps.

Consent hallows other behaviour formerly condemned as perverse, while traditional forms of flirtation are treated as threats to social order. Young men must be unsusceptible to the effects of provocative dress or abandoned conduct. They make sexual overtures, if at all, at the hazard of their reputations and careers. Should they aspire to office many years later, no statute of limitations will exempt them from challenge on the grounds of alleged youthful indiscretion. Mild banter – of a sort that might have passed as witty or at least lexically inoffensive innuendo in what I think of as my day – is effectively banned.

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Baby, It’s Cold Outside is inadmissible as entertainment. So is Sleeping Beauty. When I was teaching a course on the history of the relationship between humans and other apes, I showed the class an abridged version of the first Tarzan movie. The cry of “non-consensual!” went up when Tarzan stole a kiss from Jane: the students were joking but it was the kind of jest that discloses many a truth.

Romance used to rely on implicit, modest, maidenly encouragement. Talleyrand’s funniest joke concerned the difference between a lady and a diplomat: “When a diplomat says ‘yes’, he means ‘perhaps’. When he says ‘perhaps’, he means ‘no’. When he says ‘no’, he’s no diplomat. A lady, when she says ‘no’, means ‘perhaps’. When she says ‘perhaps’, she means ‘yes’ and when she says ‘yes’ she is no lady.” Now, only yes means yes. To be well-behaved, you have to be a brazen woman or a coy man.

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Tarzan’s kiss has become a curse and Talleyrand’s joke an objurgation. Teasing is taboo. Flirting is in flight. How has the transformation happened? Some of the explanation seems obvious. As opportunities for women multiply in work and education, so does abusive, insulting and exploitative behaviour among male teachers and bosses. Every self-interested exercise of power is evil and demands counter-measures. It is hard to free women without fettering men. Romance, meanwhile, withers as chivalry wanes: even protractedly educated males may not know how to behave decorously. As the precision of language slackens, the definition of impropriety widens. As the balance of power between the sexes shifts, equilibrium remains elusive. There are men who try to escape a sense of emasculation by coercing women, and women who relish the chance to denounce male excesses. To reimpose decent limits on permissiveness, women seem the better qualified sex.

Consent is, at best, an amoral principle: you cannot make a bad act good by consenting to it. But there seems no other practical rule to follow. Participants’ perceptions of unsuccessful encounters are likely to conflict and it is obviously unfair to make people of one sex the arbiters of the propriety of sexual conduct; but I would rather grant the deciding voice to women than to men. Without eradicating male misconduct, we have equipped women to respond with abuses of their own by misrepresenting, for instance, insensitivity as importunity, or importunity as aggression. In the absence of a third sex, however, it is impossible to shift power away from men without transferring some of it to women.

So don’t scream “sex!” in the classroom: your intentions may be misread. What would screenwriters do if they had to re-make Arabesque today? Substitute “Fire!” or “Brexit!”? Either utterance would be less dangerous and probably less effective. Gregory Peck’s audience may have to be allowed to sleep on, without an effective arouser.

Felipe Fernández-Armesto is William P. Reynolds professor of history at the University of Notre Dame.������

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