I disagree with Zoe Margolis on one matter, and so far as I can make out, only one - she thinks her story is not particularly newsworthy. She insists that, personally, she is not interesting, and politically, well, come on, there's a war going on in the Middle East, how in God's name can we be talking about a sex-blogger. I, conversely, think her story is fascinating, and the politics it rips open are not trivial, whether there's a war going on or not.

To recap: Zoe Margolis is her real name. Under her pseudonym, Abby Lee, she started a sex blog at the beginning of 2004, which is witty, moreish and incredibly explicit. She gets laid, with exes, with people she's met on the internet, with friends of friends, with people who are doing some research on, er, getting laid. It is very rare that she goes more than a couple of days without having a wank somewhere unusual. The wanking bits are the funniest, actually, especially a memorably neat definition of a "bully wank" (see the extracts from her book below). She has the odd same-sex experience, she remembers past shags and ruminates on techniques she would like to refine a bit.

As a feminist, I would like to believe, or even pretend to believe, that she is run-of-the-mill, sex-wise, that loads of women are as uninhibited and adventurous and experimental, and approach this much sex with this much joy, but I think Margolis is pretty unusual. So unusual, in fact, that when I read the book, before I had met her, I have to admit it stretched my credulity a little. Probably because I have mindlessly imbibed some sexist bilge about women not liking a lot of sex, but also because blogs so often do turn out to be hoaxes, this kind of sex diary - by which I mean, very open, and written by a woman - never feels totally real. As authentic as the writing feels, it could well be a composite of different people, or, at a push, a man with an imagination.

Meeting her, though, I buy it totally. She has a rare self-possession, she is very charismatic, she has a lot of presence, and she is very much as she describes herself in the book - kind of curvy and imperfect and sexy as anything. Everything fits: she has exactly the London semi-posh drawl you would expect a person who grew up in Hampstead to have, and nothing jars.

Anyway, response to her blog was incredible, and largely, at first, along the lines of "thank God you've written this, I thought I was the only woman doing it." I have not actually read enough erotica to know whether this counts as erotica, but Margolis says that it doesn't. "There's erotica for erotica's sake, for stimulation, but I wouldn't put my writing into that. There's too much analysis going on, and too much intellectualising of what I'm experiencing. I like porn, but I think we need to take control of it as women, and make better porn ... but that's a whole other argument. I don't write it to titillate, and I censor like crazy so that it's actually less erotic."

The blog tootled along for about six months, and then suddenly went crazy. People were Googling for it at a rate that was measurable by the minute. Girlwithaonetrackmind.blogspot.com has had more than 2 million visitors, gets 100,000 readers a month, won Best British or Irish Blog at the 2006 Bloggies, and was published as a book last week (for which she got "six figures"). The book is already in the bestseller lists. So far, so successful, but so still anonymous; all anybody knew about Abby Lee was that she worked in the film industry, lived in London and got laid a hell of a lot. Oh, and we knew that she had size eight feet - much has been made of this, in the press, as if she were deliberately trying to out herself, whereas, in fact, I can personally vouch for the fact that big feet are not all that rare.

Then, at the weekend, the Sunday Times revealed Margolis's true identity, under the rather misleading standfirst: "By day she worked on Harry Potter, but by night ..." (It's not misleading, in so far as she did work on a Harry Potter. But she is a camera operator. They were trying to make it sound like she was corrupting the child stars.) Margolis, naturally, is principally upset about the impact this is having on her life.

Apparently, it is the oldest trick in the book, but the papers finally got a picture of her by sending her some flowers purporting to be from Random House (the parent company of her publishers, Ebury). "Well, the guy made me sign for it, and lean out of the door, which made me suspicious anyway, and then when I saw who they were from, I knew it wasn't the publisher: they don't know where I live - or my name!" She packed a bag and left her house, and hasn't been back. There have been photographers in the garden ever since, taking photos of everyone who goes in or out and has brown hair. There are snappers outside her parents' house. "Did you have to tell them?" "Yes. But they have fundamentally said they will not even glance at it. Thank God. Who wants their parents reading that stuff? It's private."

Really old acquaintances of hers have been squirrelled out and offered money for stories and pictures. You can see why she thinks of it as disproportionate, but you can also read in the Sunday Times piece, and a follow-up in the Daily Mail, how radically attitudes to female sexuality have regressed in the past decade.

The Sunday Times talked about Margolis's "seedy" dates and "shameless" conquests - that would not have been an acceptable way for a broadsheet to discuss a sexually active woman in the 1980s, or even the 1990s. The piece went on to call Margolis's "an erotic version of Bridget Jones", which is not simply a bit off, it is the exact opposite of the case. Bridget Jones is characterised by being basically asexual, far more preoccupied with calories than with sex, barely even mentioning men except where they are the source of despair or a mini-break. It is like calling someone the "erotic Mother Teresa". As hard as it is for Margolis, it is brilliant for the rest of us that busting her identity should turn this into such a talking point. Are we, after all, happy to return to a time when promiscuous women could be openly derided like this? Because if we are not, it is time we said so.

Margolis herself has very little rage about all this. "I have been writing the blog for two and a half years, so I have been called every name you can think of, many times," she says. "I have built up a bit of a thick skin about it. And there's enough of a community built up that we can actually do something positive about it rather than just reverting to tired old sexual stereotypes. I'm expecting to be labelled and put down, and really attacked for my liberated sexuality. I know what they're going to say, and I can't imagine they're going to say anything that will shock me."

Her career in films, she thinks, is totally shot. "I'm a laughing stock." Why, has she shagged everyone she's worked with, and then written about it? "No! I didn't shag a load of people! Very, very few. When you work 16 hours a day, you don't have time for sex."

"I'm really sorry to hear that."

"Thank you. But the point is, getting taken seriously as a woman is difficult enough anyway, but when everyone knows the background of your sex life, and everyone's been laughing about you behind your back, how on earth are you going to make them be quiet and focus?"

Worse, though, all this will play havoc with her love life. "I'm single. Single and shagless, and this isn't going to do much for my chances. 'Oh, so you're the slapper ...' A lot of guys are intimidated by it, if they know that you've had a lot of lovers, it doesn't sit well with them. I suppose I can use the book as some sort of test - 'Just read that. If you don't have a problem with it, then you must be OK.' But then how do I know that they're not psychos from the internet? Obviously, I value my readers so much, I really do."

"And yet, you just called them psychos from the internet."

"No, no! They're not the ones I mean!"

Margolis is an extraordinary person. She seems to me to be in pretty buoyant, punchy spirits, despite all the havoc. Since the secret has got out, there have been people who have recognised themselves in the blog/book, and there are a couple of people who are pissed off, which I believe was never her intention, but her feminist agenda dovetails so neatly with her sexual one that it would be impossible for her to look at this as an entire disaster. "I did think, all along, that maybe the women who would read Elle, or whatever, would pick up the book thinking it was chick-lit, and get politicised. That was always my hope, to have a political subtext in there, packaged up. A spoonful of sugar, you know ... Feminism has such a bad reputation, and it's seen as so unpalatable. How could you get it on the front page of anything?"

She is really trenchant on the sorry-arsed, calorie-counting era that the Bridget Jones fixation ushered in. "Without attacking the writer, when I flicked through that originally and saw that there was all this obsession with weight, I just didn't relate to it. I know so many women focus on that, but it just isn't a priority for me. And I draw a correlation between being sexually uptight and certain eating disorders. Because, literally, they can't let anything pass their lips. They can't enjoy anything, I can't imagine them enjoying sex. I can't count the number of friends I've heard saying,'I'd rather be underneath in bed because then he won't see how fat my stomach is.' That's the last thing you should be thinking about while you're having sex!

"It's not coincidence that when you look on any shelf in the newsagent, [at] all the women's magazine covers, everybody is either too fat or too skinny. Jesus, can't we think of anything else as women? And all the men's mags are 'fuck this bird!' For God's sake, the guys are sexualising, and the women are having control issues with food. It's just obscene."

Being uninterested in going on a diet is not Margolis's only heresy. She is against plastic surgery. ("It's not advancing women, is it? We're still not being paid equally and not reaching positions of prominence. And if it's just about vanity, then I'm not interested"). She laughs in the face of chastity as a virtue. She thinks women's magazines are touting ideas that are honestly dangerous to young women. None of this is radical, in the grand scheme of feminist thought, but it is all radical now - even not being on a diet is radical now. Not being on a diet and sleeping with whoever you fancy and objectifying men, not just waiting to be objectified - all this at the same time is incredibly radical now. It is no surprise at all to me that Margolis gets young women writing to her and saying she has rocked the way they look at themselves. It is no surprise at all that she is providing a view and a voice that they would not get in the mainstream media. I think she is the voice of third-wave feminism. She disagrees. "I'm not well read, I'm not well educated, I don't have a library at home with feminist literature that I could quote off the top of my head. All I have is my experience."

I tell her that I think she has done an amazing thing, and that I am really jealous; that I wish I had done it. "Well, you should have shagged a few more people," she says.

Extracts from Abby Lee's book

Sunday January 9

Blog Boy has returned my email! And, I am happy to say, is flirting back with me online. More importantly, he has agreed to meet me for a drink next week, and I'm overjoyed.

There is something about the way this guy writes that I find utterly captivating; I have to get to know him. I probably shouldn't get my hopes too high, but I can't help being excited about seeing him in the flesh. I can't help but wonder what he looks like naked.

Monday January 17

For almost an hour I was on the brink, but I couldn't risk orgasm-induced temporary blindness on a dual carriageway, so when I finally got home, I bolted inside, kicked off my boots, dragged down my trousers and my thermals, and tugged down my pants.

And do you know what happened? For all my randiness, the 5am starts, the long day and the sleep deprivation finally got to me and all I could manage was a bully wank. This is where you've mildly got the horn but you're tired/had a long day/been wanking all day anyway and then you force yourself to masturbate on top of all that. Of course, consent is involved, it's not like you have to coerce yourself into grabbing your own genitals - "Oh come on, honey, you'll like it, I promise!" - more that you feel you should have a play, even given the state you're in, and you really try to enjoy yourself, but you heart's just not in it.

Thursday January 20

Got a sweet text from Blog Boy checking we were still on for dinner when I get back from filming next week. His message was like a little shot of joy: I was standing outside for 10 hours in the freezing rain today and knowing I have a date with him to look forward to really cheered me up.

Friday January 21

I feel like a woman again. Finally I got some sex! I just couldn't wait any longer, despite all the texts from Blog Boy - my willpower had evaporated. And what a shag it was. Last night, Tony and I arranged to meet up after work in the hotel bar. We drank some wine, made some idle chit-chat about our day on set and then moved the conversation on to sex, Tony saying he hadn't had any for a while ... It began to get late and, with an early start this morning, we made our way to our rooms to go to bed, Tony stopping off at mine (supposedly) to "compare" the room with his. It didn't take long for things to happen: within minutes we were down to our underwear and were frantically grabbing at each other.

I finally got naked with a man for the first time in months.

Wednesday February 16

I have an addiction. I tell myself I won't let it get the better of me, but it is officially out of my control. No, not the sex one. The other one. You see, I now have almost one hundred items of underwear in my drawer ... I'm not sure when this addiction started. For years I wasn't into wearing anything saucy, because I thought I would just be perpetuating the same sexist, objectified view of femaleness that was shoved down my throat by the cover of every magazine.

Back then, I thought that wearing lingerie represented the male fantasy of female sexual availability, so how could a feminist like me wear something that seemed to exist just to turn a man on? Plus, the thought of my partner getting off on me wearing frilly underwear made me extremely uncomfortable, so all my early relationships were spent wearing comfortable knickers and sensible bras. Bridget Jones had nothing on my big pants.

At some point in the past couple of years I began to find lingerie appealing. I started to enjoy looking at it and touching it, and when I held it against my skin, it made me feel seductive. The biggest shock came when I finally slipped a lacy G-string up my thighs and the sight of the curve of my arse against the material turned me on ... And I found out that it also turned me on to know that I could turn on a partner, too: I like the idea that a man could enjoy a thing that gave me so much pleasure - even if it was the thought of me in a boned basque and stockings. I didn't feel degraded by this - I felt empowered.

· This is a (heavily) edited extract from Girl With A One Track Mind, by Abby Lee, published by Ebury Press (£7.99)