John Walsh: 'I was defending the most testosterone-fuelled bloke in 20th-century literature'

Some works of literature are familiar to readers for only one detail, or line: Many know Shakespeare's The Winter's Tale, not for the climactic redemption scene, but for the stage direction, "Exit pursued by a bear." War veterans may know nothing of Milton's works, but they'll know his sonnet "On His Blindness" for its final line, "They also serve who only stand and wait." The Victorian "novel of sensation", East Lynne by Ellen Wood, is known only for the line, "Dead! Dead! And never called me mother!" which didn't appear in the original book, only in a stage adaptation.

Last Saturday, at the Cheltenham Literature Festival, I found myself in a heavyweight debate, featuring a classic only-one-thing-known-about-it book. The event was a spoof Booker Prize, in which a panel of bookish geniuses, ahem, discuss what novel should have won the prize in a certain year, eg, 1848: would it have been Vanity Fair, Wuthering Heights or Dombey and Son? We were discussing 1969. I was singing the virtues of Philip Roth's Portnoy's Complaint against the considerable firepower of Kate Adie (supporting Graham Greene's Travels With My Aunt,) Mary Beard, professor of Classics at Oxford, on The French Lieutenant's Woman, and The Times' formidably clever literary editor, Erica Wagner, on Margaret Atwood's first novel The Edible Woman.

Three books, then, about a Woman, a Woman and an Aunt, plus a fourth, about a Bloke – perhaps the most egregiously testosterone-fuelled bloke in 20th-century literary history. Alexander Portnoy is a morally focussed, 33-year-old Commissioner for Human Opportunity in the City of New York, a doughty champion of the oppressed. But he can't maintain a dignified grown-up liberal-activist stance for long without the memory of his Jewish mother driving him nuts with her possessive nuttiness.

As though in retaliation, the teenage Portnoy becomes a compulsive, and inventive, masturbator: he forgets himself with a baseball, with intimate garments from the family laundry, with a cored apple – and a pound of liver he buys for the family's supper.

That's the one detail everyone remembers, or knows, about the book. I could go on until dawn about Roth's hilarious take on non-Jewish culture, the problem with non-Jewish girlfriends and the trauma of visiting Israel – but it wouldn't make a difference. Everyone knows Portnoy's Complaint is about some awful Yank (perhaps not le mot juste here) jerking off into butcher's-shop offal, and that's all they'll ever want to know about it.

"Tell me it's not true, John," Professor Beard said, when we met in the Green Room. "Tell me not all men are like that." I tried to explain that Portnoy is a character, not an actual trembling-handed bloke in the street, and that in fiction you're allowed to ... But I'd lost her. Kate Adie shuddered with distaste about Portnoy's teenage hobby – ignoring the character's more considered relationships – and praised the Lord it could never be filmed (it was, by Ernest Lehman in 1972). The audience was asked to vote for what the outcome would be. Most voted for John Fowles or Graham Greene; a mere smattering of hands was raised for Portnoy, as though we were a conspiracy of furtive manipulators, sharing a grubby secret.

I ranted about the book's humour, satire, political rage and social embarrassment, but it was hopeless. Mary Beard shook her head at my becoming a prey to such filth. Kate Adie regarded me sternly as if it were a disgusting phase I was going through. The audience sounded as if, like Robert Benchley, they wouldn't mind meeting the author of Portnoy but they sure as hell didn't want to shake his hand.

Then the damnedest thing happened. Erica Wagner (a New Yorker) decided the Atwood novel was too amateurish, and thought Portnoy should win because it was funny and clever and a real achievement. I blinked to find that anyone (or do I mean "any woman"?) was able to see beyond the handjobs and the liver. And for the rest of the debate we sat there, Erica and I, fighting off the opposition. The audience's final vote elicited about five votes for Portnoy. Prof Beard stuck with The French Lieutenant, Ms Adie stuck with Travels With My Aunt – and the Portnoy faction won the debate, in the teeth of the audience's disapproval. It was a notable victory. But I was amazed nobody hurled the inner organs of beasts and fowls at us, as we left.