Don't come here if you don't have A levels ... the Arcola Theatre's production of Lysistrata. Photograph: Tristram Kenton

A month or so ago, on a blog I wrote about Mark Twain, a poster called liberaldogooder made the probably irrelevant, but definitely amusing point that: "it's possible to be a total literary conservative and spend your days reading about sex, violence and torture..."

Liberaldogooder's quite right. It's a glorious and indisputable fact that the western canon - the literature that time and the mysterious invisible authorities that decide these things have deemed worthy of preserving - is full of high-grade rudery. The naughtiness of supposedly "nice" books has been an open secret among educated readers for millennia. Sex and scatology were a significant part of the literary establishment even before Aristophanes strapped phalluses on to his actors and made them recite gags about farting.

Since then, Catullus told has told us just what kind of kisses he'd like to give to Lesbia, Ovid has given impish advice on the art of love and, to use two of liberaldogooder's examples, Petronius filled the Satyricon with castration jokes and Apuleius made his ass see a decidedly seamy side of Roman life.

The saucy tradition has continued uninterrupted since classical times, running on from Chaucer's bawdy rhymes to Anais Nin's sophisticated erotica via Andrew Marvell's wonderfully florid attempts to get his coy mistress into the sack. And it's only a matter of time until an American evangelical works out what Shakespeare is really on about and tries to add him to the long list of books and plays already banned in various parts of the USA.

Now, I'm happy to admit that that this blog is basically an excuse to provide lewd links so we can giggle like schoolboys who've just discovered that dictionaries contain rude words. After all, my giggling schoolboy days were some of the happiest of my life. I'm also aware that it may be slightly ridiculous to try and bring a serious point into all this talk of "country matters" and "fututiones".

However, there is an issue that troubles me. Mainly, I'm thinking of the old snobbery that says it's perfectly alright for Chaucer to talk about "quaints", planting kisses onto a naked "ers" and letting "fle" farts, but should DH Lawrence mention the c-word, that's a problem because the lower orders might get hold of it. Yes, the Lady Chatterley's Lover trial seems like a distant, ridiculous memory today, but the vestiges of that condescension still remain.

Call it smut and it's cheap and shameful, call it erotica and it's all a jolly good laugh. Put a sex scene in a "literary novel" and it's a serious attempt to grapple with one of the most intimate facets of the human condition. Pad out your blockbuster with bonking and it's trash. If Ian McEwan fills healthy portions of his books with awkward young men's masturbatory fantasies, it's art. When Jilly Cooper glories in no holds barred rutting, it's a cheap attempt to shift units and titillate her (by implication rather pathetic) readers.

By the same token, if you write a serious, difficult book like a Clockwork Orange that only the most literate are likely to appreciate, then the rape, murder and pillage constitute a fascinating intellectual exploration. Transfer this ultraviolence to a film that any old mug from the street can go and see and it's dangerous...

OK, I'm over simplifying, but I don't think we've entirely escaped from the assumption that clever people can cope with raunch and others, well, it might give them bad ideas! Or perhaps you think there is a difference between pornography and artistic erotica and that one deserves more respect than the other, elevated as it is by art. Either way, comments would be welcome. Alternatively, of course, just name your favourite bad bits in the best books. It should make Friday afternoon more fun, after all.