Nishi springs into action, weaving through the crowds like an eel. "Excuse me," he says by way of introduction, "I have a client who really likes the look of you. He's over there in front of the restaurant. Would you like to meet him?" When the women protests, Nishi presses his case. "He's rich - a rich salaryman - and he'd like to buy you a present. Come on, just meet him. You don't necessarily have to go to bed. Thank you. Thank you very much."
Slowly, and still somewhat reluctantly, the woman finds herself falling into step. They walk back to the salaryman and Nishi effects an introduction. Three hundred pounds richer, he then disappears. Two minutes later, so do the salaryman and his new conquest.
Sometimes, these strange meetings end in "love hotels", rented by the hour with condoms provided. Sometimes, perhaps more often than not, they end in regret, with one or both parties wondering why it happened, what impelled them to do it, and whether they would do the same again.
When I began work on my new book and television series about human sexuality, Anatomy of Desire, this seemed to be a genuine puzzle. My assumption at the time was that men were designed to spread their seed as widely as possible while women were built for monogamy. Ten thousand years of history seemed to tell us as much, not to mention the bold new theories of the evolutionary biologists.
Men can produce three million sperm a minute, the biologists continue to tell us; since we are all designed by the process of natural selection, where the winners are those who breed the most, surely men should be biologically driven to use as much of that sperm as possible. Women, on the other hand, who have a very limited number of eggs and for whom every fertilisation ushers in a draining nine-month period of pregnancy and many more years of care, should be concerned not with quantity of sexual partners but with their quality. They should seek the best possible mate they can, the argument runs, then hold on to him for dear life.
This, however, is not quite the case. First, the evolutionary biologists often fail to take note of the enormous cultural and personal pressures foisted on men and women. Over the past three years, since beginning research on the book, I've met women who were sexually interested only in radio tape recorders and men who were aroused by almost anything except women: dogs, children, even corpses. All these people were extremely heterogeneous but they usually had one of two things in common. As children, they had been either catapulted too early into the world of adult sexuality, usually as a result of having been abused by a male relation, or their early sexual feelings had been too radically suppressed.
In households with fundamentalist Christian convictions, the results were particularly severe. Here, where sexual feelings (especially sexual feelings among children) were considered shameful and dirty, punishment was often the surest route, not to the suppression of those urges, but to their redirection.
Mark Matthews, caught in bed with his young cousin at the age of 12, recalls how he was dragged from under the covers, thrashed by his mother and told how "I was evil, I was dirty, and that touching girls was a terrible sin and would make me grow up to be a crazy man who would be locked in a crazy house and that I should never try to do that to a girl ever again in my life". He didn't. Instead, he climbed into fields late at night and did it to horses until horses, rather than women, became his chief source of arousal.
I don't mean to suggest that Christianity is doomed to turn us all into hardcore paraphiliacs. Far from it; in its milder form, with its emphasis on kindness, forgiveness and the sanctity of marriage, it has often forged a more constructive path towards personal and societal stability than many of the world's other major religions. I merely mean to illustrate how any cultural force, be it religious, legal, or moral can - if meted out in sufficient doses - rope in and redirect the sexuality of its citizens. For most of us, living in the post-Christian west, the ideal of monogamous marriage combined with the church's traditional scepticism about sex has surely led many to be more circumspect about engaging in extra-marital affairs than those living in more liberal climes.
Yet the evolutionary biologists are not just mistaken in their underestimation of the shaping powers of culture and upbringing. They also make a mistake which seems to me firmly biological. And the Nishi scenario sums it up perfectly. On one level, the Japanese salaryman seeking a short-term encounter with a stranger on the street is the biologist's dream - proof if proof were needed that he's doing everything he can to spread his seed, metaphorically if not literally. Yet the most interesting thing for me is not that the salaryman is going behind his wife's back to seek with strangers; it's that he's got a wife in the first place.
Wives, as anyone who's got one will testify, take time and money. They generally don't like their husbands cheating on them; they require their husbands to be at home for a considerably large portion of their free time, and they can act as a powerful anaphrodisiac for other women who well might, in different circumstances, be perfectly happy to bed with their husbands.
For men hellbent on spreading their seed, wives seem like an altogether bad idea. Bad, that is, until you see the situation slightly differently. The point about natural selection is not that it's a race to reproduce as widely as possible; it's a race to reproduce as well as possible. There's no point in a man siring thousands of children via one night stands if none of those children reach adulthood and have a chance of reproducing themselves.
How much more skilful, strategically, to marry a decent woman, make sure you know that all the children she conceives will be yours, then pour large amounts of energy and attention on those children to make sure they grow up as happy, healthy and reproductively viable as possible - and then spread it around on the side as well. Men have inherited a dual tendency which is much more subtle, and also much bleaker, than most evolutionary biologists believe. They're not natural polygamists; they're natural monogamists, with a passion for adultery.
So where does all this leave women? It sounds like a no-win situation. Every man wants to get married but no man wants to be faithful. Let's go back to the idea about reproducing well. For women, who can have only a comparatively small number of children, this means finding the best possible mate whose attributes will be combined in the most robust possible children. She needs only one because he's quite capable of giving her all the sperm she needs - and then some. What she's looking for, I believe, is the holy trinity of male attractiveness: good genes, which can be passed directly to the child; high earning power, so that the child can be given the best possible start; and long-term fidelity, so that the fruits of the husband's love and labour will not be transferred to someone else.
Unfortunately, as so many women today lament, it's very difficult to find the whole lot in one package. Desired by so many women, the best quality males are even more unlikely than the rest of us to confine themselves to one. From the despots of the early civilisations through the politicians and pop stars of today, powerful, handsome, resource-laden men have traditionally had more sexual partners than most people, simply because they can. In the face of these odds, the woman's evolutionary ideal of a life of tender loving care from a single perfect man would seem doomed.
Doomed, that is, unless it had been backed up by some rather more practical fall-back strategies. For I believe that evolution, the ultimate pragmatist, has actually taken women down a rather different path.
Let's return to the sexhunter scenario. The salaryman was married. We know that. But what about the woman he picked up? Mr Nishi, in an interview with our series director Richard Curson Smith last year, revealed that a large percentage of the women he takes back to meet his clients are in fact married as well. If the problem, from a male point of view, is why get married in the first place, the problem from the female point of view is why have an affair. Your husband at home will provide you with all the sperm you need. Then why risk playing away?
The answer, it seems, is this. Unable to combine the holy trinity of good genes, high earning power and long-term fidelity in a single partner, women too have been designed to hedge their bets, marrying a safe, reliable man who will protect and provide for them, then reaping the benefit of sex with more dashing partners behind their backs.
In a recent study of married women who visited nightclubs on their own, a large preponderance were found to do so on the very nights when they were most fertile. Not only that, they both dressed and behaved in a more provocative fashion than their non-ovulating or unmarried counterparts. And what sort of men were they looking for when they were there? Most wouldn't tell, but in studies of those who do, by far the most important criterion is good looks. Having set up a permanent flow of resources from Mr Nice-but-Dull at home, women are on the look out for good genes from Mr Tall, Dark and Handsome.
In a sense, it all sounds unbearably cynical. But evolution, "red in tooth and claw", never awarded marks for second place. Moreover, look at the DNA studies that have been done in the UK, and one fact leaps off the page straight away. Between 10 and 12 percent of children born in the UK are not the genetic children of those who believe they are their fathers. We are a nation of bastards, often literally.
Simon Andreae's book, 'Anatomy of Desire: The Science and Psychology of Sex, Love and Marriage', is out now (Little, Brown pounds 16.99). The accompanying television series starts tomorrow at 10pm on Channel Four.