Fifty Shades of Grey is easy to mock. The reason it's easy to mock is that it's porn. I picked up the book, with its dark-and-mysterious cover that looks, through half-closed eyes, a bit like one of the Twilight novels, in an airport. I read it on the plane, and I enjoyed it. There, I said it. I enjoyed it because there were, amongst some terrifically trashy bits of girly romance and some eye-watering blow-job scenarios^[1], a few quite good, quite detailed descriptions of fucking written from the point of view of a woman who seemed to be really enjoying herself.

That's it. That's all. Fifty Shades of Grey is porn, and porn can be quite fun. With the publishing industry in such choppy waters, I fail to understand why this record-pounding paperback has come in for extra-special derision all over the world, other than the fact that some people are appalled at the idea that somewhere out there, well over ten million women might be – whisper it – masturbating.

"But it's badly written!", I hear you cry. Um, hello? It's PORN. Whilst there is some pornography out there written with a deft stylistic hand – from Anais Nin and Henry Miller to Anne Rice's luscious, filthy Sleeping Beauty series – that's hardly the point, even if you don't buy Oglaf [2] author Trudy Cooper's adage that "erotica just means porn that works for me." A dildo painted with an intricate lubricant-insoluble motif may look delightful, but a plain old rubber shocker gets the job done just as well. This book is porn. It is for wanking to. Pornography made for men is rarely judged on its artistic merits – the average 20-minute RedTube clip has hundreds of thousands of views and practically nobody leaves comments complaining that the lighting is garish, that the pounding cheese music is weird and unsettling, or that there's someone's Bassett hound running about in the background^[2].

Similarly, I can't recall Page Three of the Sun ever getting taken to pieces for its lack of artistic imagination. The point, the only point, is to show three million men some tits in the morning, and they've been happily ogling those pixellated teenage breasts on public transport for thirty years. That's understood. Exactly the same basic principle applies to the Fifty Shades series, which has the added bonus that no actual nubile, desperate postpubescents were harmed in its production – but somehow the idea that women might gobble down a poorly-written book in their tens of millions just because they've heard there might be some fucking in it is uncomfortable for the sort of snobbish commentators who have absolutely never themselves bashed out a cheeky one over FHM magazine.

When you get down to it, the problem most people seem to have with Fifty Shades of Grey is that it's for girls. Even worse - it's "mommy porn", porn for mommies, for older women to read and get excited about, and that dangerous nonsense really needs to be stopped right now. Everyone knows that the only women who are allowed to actually have sexuality are slender, high-breasted twenty-one year old virgins – rather like, it has to be said, the heroine of "Fifty Shades of Grey".

Tens of thousands of words have been wasted over whether Christian Grey, our well-tailored, long-dicked hunk of fictional man-meat, is an appropriate lust-object for today's right-thinking feminist, but less attention has been paid to the fact that Anastasia Steele, the protagonist, rather embodies the contemporary concept of "fuckable". Those of us reading Fifty Shades may not all be innocent virgin college graduates, but getting moistly involved with a hardcore sexual fantasy feels less uncomfortable if you can temporarily imagine that you are. Virgin college graduates don't have to feel guilty for fantasising about being seduced by a gorgeous young multi-millionaire entrepreneur with his own private jet and a fleet of audis who's rather unnervingly like Mark Zuckerberg, if Mark Zuckerberg were hot and well-dressed.

Derivative and aesthetically childish though they may be, women everywhere are reading these books, especially now that ebook technology uptake has reached a point where anyone with a smartphone or Kindle can read porn privately on public transport, or one-handed in their bedrooms. The only people who haven't bothered to read the damn books, it seems, are most of the journalists writing about it – which seems to be the only possible explanation for why the parts of the series that have been most anxiously discussed are also the least interesting.

Firstly, there's the sadomasochism. Katie Roiphe's now-infamous Newsweek cover story claimed that the popularity of the Fifty Shades books was evidence that women everywhere are tired of all this feminist liberation and secretly want to be tied down and whipped by wealthy plutocrats. But in fact, there are barely two spanking scenes in the whole of the first book – by far the most in-depth and detailed sex-scenes are "vanilla" – and our protagonist spends most of the time feeling shocked and horrified about her paramour's predilictions, to an extent that anyone actually involved in the S&M community might well find offensive. The watered-down approximation of sadomasochistic sex in the first book, at least, is merely an extended fantasy of possession, of being utterly desired by a person who takes full physical, moral and social responsibility for any boning that may or may not ensue. In a world where women are still made to feel ashamed of ever wanting to experience sexual pleasure for its own sake, that's an appealing fantasy.

Secondly, and most importantly – these books started out as smutty fan fiction. The publishers are extremely keen to underplay this aspect of the Fifty Shades books, and E. L James doesn't discuss it in interviews, but the fact that these books began as extended stories published on the internet in the Twilight fandom community is, to my mind, the most fascinating aspect of the whole Fifty Shades phenomenon.

If you're not familiar with fan fiction, or "fanfic", please just take my word for it that there are countless thousands of men, women and girls out there on the internet – mostly women, mostly young women, and some of them extremely young women – writing and sharing long, dirty stories set in their favourite fictional universes, from Harry Potter to Buffy and Twilight. These stories tend to place beloved characters in sweaty pairings that make private sexual fantasies a community experience – readers comment on and critique one other's work, correcting the most anatomically implausible details and discussing the ins and outs and ins and outs of possible scenarios at breathless length.

Not all fan fiction is filthy, but a great deal of what makes the enormous volume of dirty short fandom stories out there on the internet so exciting is that it's a unique way for readers to re-occupy a text, to rewrite anhedonic, sexless sagas like Harry Potter or actively disturbing chastity propaganda like Twilight with all the bonking and bodily fluids back in. Dirty fanfiction existed before the internet, but online forums have allowed enormous communities of antsy fifteen-year-old girls to crowdsource the education their classmates are getting from RedTube. It was in one of those communities, written largely by women, largely for women, that Fifty Shades emerged, and that fact probably goes quite a long way towards explaining why it works so damn well as what it is. Which, to reiterate, is porn. For women. To masturbate to. Horrifying, I know, but I suggest we all get used to the idea.

[1] "My very own Christian Grey-flavoured popsicle" is not a phrase I'm going to be able to burn out of my brain any time soon.

[2] For more on this theme, visit the absolutely genius indifferent cats in amateur porn tumblr, which just goes to show that the oceans of human time lost in the lonely, backlit wank-alleys of the internet have not been entirely wasted. If you're under 18, get someone who isn't to Google it for you.