Howard Jacobson: Men – once you run out of fingers to count your sexual conquests, it's time to stop

What happens in the dark is not for gossiping or bragging about in the light

To hell with all our troubles, let's start the day off with a song. "In Italia seicento e quaranta;/In Almagna duecento e trentuna." You know the rest: after 640 in Italy and 231 in Germany, there's 100 in France, 91 in Turkey (Turkey!) and 1,003 and still counting in Spain. No mention, notice, of the United Kingdom, where either the women were too chaste or Don Giovanni couldn't face hanging around for between 50 minutes and three hours at border control – depending on whose figures you believe – waiting for his passport to be stamped. Not someone you think of as a queuer, Don Giovanni.

So what about you? How many people have you slept with? Forgive the intrusiveness of the question but there is such a fashion for bragging about one's sexual conquests – especially, it pains me to say, among men – that I'm assuming you're happy to brag a bit yourself.

First, it was William Roache, otherwise known as Ken Barlow of Coronation Street, telling Piers Whatsisname that his fellow actors nicknamed him Cock Roache on account of... well, that's not hard to nut out, but just so there should be no mistake, on account of his having slept with 1,000 women – a figure so round you wonder if he'd been aiming for it and stopped the minute he got there. Tough if you happened to be the thousandth, in that case, wondering why the inordinate celebration: champagne, punching the air, shouting "Whoo!" and then the sudden deflation.

In fact, considering that the "Cock" turned 80 last month, 1,000 is not really all that many. Allow him to have been sexually active from the age of 16 and that means he has slept with only 15.625 women a year, which comes out, if I've done my sums right, at 1.302083 recurring a month. Chickenfeed. So what was he doing on the other 29.697917 nights? Learning his lines?

And now, not wanting to be outgunned by a mere soap star, the disc jockey and human emoticon Tony Blackburn claims a haul of 500 which again, no matter that he's a decade younger than Roache, strikes me as hardly worth shouting about. The number of hours I have never expended listening to Tony Blackburn must run into the hundreds of thousands, but you won't catch me boasting about them.

Of the host of questions there are to ask about all this, the first is: "How do they know?" According to a two-page spread in the Daily Mail, Engelbert Humperdinck has had 3,000 lovers, Charlie Sheen 5,000, Fidel Castro 35,000, Nick Clegg 30, Warren Beatty 12,755 "give or take". Are those real figures or stabs in the dark? Either way, how is it that men who like to think of themselves as lovers don't know it's the height of erotic unsophistication to keep count? It's one thing for Don Giovanni to let his servant Leporello do his calculations for him, but he would go down in our estimation were he to enumerate his successes himself.

Urbane seducers of either sex are indifferent to the number of lovers they have had; they are too busy to keep count, too absorbed in the present to remember the past, and too polite to discuss either. Essential to any idea of civilisation is the principle that what happens in the dark is not for gossiping or bragging about in the light. "The heart must have its secrets," D H Lawrence said. And whether or not the heart was ever involved in Peter Stringfellow's 2,000 or Mick Hucknall's 3,285, the body must have its secrets, too.

And there is a further argument. It demeans. Not only does it demean those slept with, it demeans the sleeper who becomes reduced to a mere vulgar fraction himself. "Was that all I was doing?" the accumulator must somewhere along the line ask. "Did I expend spirit and waste shame merely to become one-half of an equation?"

I say "somewhere along the line" but this thought should kick in pretty soon after sex begins. So at about 16 or 17, for those born before 1964, and around about the time they would once have been taking ballet lessons, collecting cigarette packets and studying for their 11-plus for those born later. Yes, you go from an ignorance which you fear will last for ever to a knowledge so liberating you feel you must commemorate it somehow and, short of a party at the Ritz, boasting and totting up is as good a way as any, but the minute you start running out of fingers to count on, you recognise yourself for a fool. I see that as one of the milestones of maturity: the hour you become contemptuous of yourself for keeping score.

We are not prim in this column. We don't think one sexual partner for life should be enough. We would even legislate against it. "Och, it hardens all within and petrifies the feelings," wrote the philanderer poet Robert Burns, in non-philandering mode, about the consequences of "roving illicitly". But not philandering can be costly, too. Of the rocks on which the best and longest of marriages can founder, the rock called Ignorance and Wondering is the cruellest. Better, any time, since every choice we make implies rejection of another, that when we choose licit love over illicit roving we do so having known what illicit roving was like.

Don Giovanni is a great opera for many reasons, one being that the Don himself is, morally and musically – if that's a distinction worth making in opera – at one and the same time hero and villain. He could also have shown for a fool, were it not for Leporello who both points his folly out and absorbs it in appreciation. The wonderful comic song with which we began this column is a celebration of his master's sexual appetite and resourcefulness, full of zestful re-creation and, of course, envy, but it also mocks the madness of it, as one might mock a man whose ambition is to eat the world. Whoever would be celebrated for the amount of sex he has had should get himself a Leporello. He should also remember that a hero in art is often a clown in life.