It's sad, but undeniably true, that our rubbish tells more about us than our art. Take the literary sensation of our times - you must have heard about it. It's called Fifty Shades of Grey, a poorly written, passingly pornographic novel that has become the fastest selling book of all time. Its puerile plot and kinky sex has sold 10 million paperbacks in America, a million E-books on Amazon; Hollywood stars are fighting to appear in the movie. The author, a British woman going under the name of E.L. James, is a squillionaire having already outpaced J.K. Rowling, Dan Brown (an even worse writer), the Koran and the Bible.

It's invented a literary genre, 'Mummy Porn'. The women who buy it are predominantly in their 30s and 40s. An entire female generation seems swept up in a story of a young virgin submitting to lots - and lots - of sado-masochistic sex at the hands, and the instruments, indeed, of a post-modern Marquis de Sade.

Feminists are beside themselves trying to explain how this could happen; how women, the powerful, independent women of the 21st Century, should be so fascinated by female powerlessness and subjugation. One suggestion - that the fantasy of submission is a welcome escape for women who are now taking care of everybody else ("the relentless responsibility of the modern woman's life", as Newsweek put it) - lit the blue touchpaper of feminist sensibilities. Another - that women have always loved porn (or, in these non-judgemental times, 'erotica') and E-books, Kindles and so on mean they can do so without others noticing, has also had short shrift.

There must be more to it, and, of course there is. Even I, a bewildered, elderly male whose notions of pornography involve strapping girls with beach balls, dimly discerns it.

Take the characters. The anti-hero, the whip wielder, is hardly Sir Jasper. He's an Adonis, we're told over and over again, who's a piano prodigy and wine connoisseur who's made his billions out of renewable energy - ha! - and is desperate to feed the Third World, right on! Sure, he knocks her about, but rubs in baby oil afterwards. New monster or what?

The tiresome virgin (you don't have to be a sadist to have the urge to slap her) is actually in control... negotiating the details of her degradation, even drawing up a contract to define their sexual terms of trade.

Her monster has it all. He's solicitous, caring and only cruel because he is damaged. So he's needy, as well as filthy rich. He's a project and you just know the virgin will change him into the tractable family man all the readers of the book are, apparently, trying to escape from. It's tripe, but it skewers that particularly paradox more neatly than Shakespeare.

But what does it really tell us? That the threshold of what shocks and/or excites us has risen immeasurably in less than a generation. That the arguments over the middle ground of pornography are simplistic. Objectifying women? Doesn't all fiction objectify people? Women are not necessarily victims, on or off the page.

Most of all, that women may be mistresses of the universe but their literary taste is no better than men's.

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