Philip Larkin did not think that sexual intercourse literally began in 1963, between the end of the Chatterley ban and the Beatles' first LP.
What he meant was that a new attitude towards sex sprang up at that time, freeing it from shame on the one hand and the strict requirements of marriage on the other.
Similarly, the Oxford historian Faramerz Dabhoiwala does not actually believe that sex originated at some time in the 18th Century.
What the title of his new book -- The Origins of Sex -- refers to is the rise of a new set of assumptions about sexual behaviour. If the foundations of modern physics were laid by Isaac Newton, the basis of modern ideas about lust and seduction was established by an extraordinary roll-call of people from Newton's period onwards: journalists, courtesans, rakes, novelists, philosophers and social reformers.
The new assumptions included the idea that sexual desire was a natural drive; that sex outside marriage was a fact of life; that consent could play a key role in the morality of sex; but also that women were naturally more "virtuous", so that the standard pattern involved the seduction of innocent women by men.
This led inevitably to double standards: men who slept with many women were just having their fun, while women who slept with more than one man were whores who had abandoned their womanly nature.
Putting this story of sexual revolution alongside the Larkin poem, we hit an immediate problem: if, as it now seems, sexual intercourse began in the 18th Century, what are we to make of the oppressive Victorian sexual morality that loomed so large in Larkin's youth?
This book does show, almost in passing, how the Victorian attitudes developed from the 18th-Century ones. Modern readers may snigger when they are told that Gladstone would prowl the streets of London late at night in search of "fallen women" to redeem; yet the obsession with fallen women was not only a central feature of Victorian moralism, but a direct inheritance from the 18th century.
Prostitutes play a large role in this book. Dabhoiwala charts the emergence of new ideas about them, noting the appearance in the 1740s of "that most enduring of modern fictional archetypes, the tart with a heart".
High-society courtesans became national celebrities, and managed their own fame in an uncannily modern way: in one publicity stunt, Fanny Murray put a £20 note (think £2,000 in today's money) in a sandwich and ate it, to show her contempt for the paltriness of the sum.
But more tolerant attitudes to prostitution went hand in hand with more determined efforts to "save" women from it, on the new assumption that they were inveigled into it against their better nature. In the 1750s two new institutions opened in London: the Magdalen Hospital, which was a reformatory for penitent prostitutes, and the London Asylum, where poor girls deemed to be at risk of seduction were sent to be trained as servants.
So-called Magdalen Houses quickly spread through the English-speaking world.
Dabhoiwala features the Dublin Magdalen Asylum for Protestant women opened in Leeson Street in 1765, the first in the country.
This is a marvellously rich and thought-provoking book, drawing on a huge range of materials, from sermons to pornography to social statistics. These too can surprise: in 1800, for example, 40pc of women getting married were pregnant as they did so.
But one might almost say that the great strength of this work -- its openness to factors of all kinds, involving changes in society, culture, literature and philosophy -- becomes a problem when it comes to pinning down the actual cause of the revolution in sexual attitudes.
Dabhoiwala admits that the explanation seems "over determined"; in other words, the answer is "all of the above".
Another problem is that while some of the factors Dabhoiwala invokes are peculiarly English, the change in sexual attitudes seems to have taken place in much the same way in the rest of Europe during the Enlightenment period.
My final doubt concerns the picture he paints of the pre-revolutionary state of affairs. Previously, he argues, women were seen as the lustful, voracious sex, and it was only in the late 17th Century that the tendency of men to seduce women was brought to popular attention.
I don't doubt that for centuries, theologians and medical theorists had written about the inferior, less rational nature of women. But there may still have been a difference between what the intellectuals wrote in their studies and what the people thought in their alehouses -- particularly when, after a few pints too many, one of their friends had a go at the serving wench.
Human behaviour does change over time; but that bit of it, surely, wasn't invented in 18th-century England.