Sex and the modern girl: Are we witnessing a new age of female sexual assertiveness?

Paloma lives in the basement of a tall, regal-looking building on a smart street in west London. Hers is a spacious studio flat with a neat patio area, just big enough for a potted herb garden and wrought-iron table with matching chairs.

Inside, the living quarters feature buffed wooden floorboards and neutral walls; there are rows of reference books, Penguin Classics, National Geographic and Vogue. Below these, taking centre stage on the mantelpiece, is a framed graduation photograph. It's nothing extraordinary, just a typical post-ceremony portrait of a slightly startled looking twentysomething with a fixed grin, clutching a rolled certificate. But in its present context, the image of Paloma with chestnut ringlets spilling erratically from under a mortarboard is somewhat incongruous.

For today – some five years later – almost directly below where the photo stands, the former Economic History student is squatting in the middle of her living room carpet, rummaging through a suitcase stuffed with lace garters, feathered face masks and beaded nipple tassels. Paloma pulls out a silver sequin suspender belt which she hoiks over faded denim jeans: "What do you think?" she teases, giving a flamboyant twirl. Her friend Camilla, a long, sinewy creature who is curled up on the sofa, looks up briefly: "I think you'll be the belle of the ball, babe," she purrs, before returning her attention to painting her fingernails.

Moments later, the doorbell rings. "That'll be the girls!" shouts Paloma, as she skips off down the hallway before returning with Claire, a smartly dressed brunette, and Bianca, an elfin figure with long auburn hair. "Sorry we're late!" they coo, offloading a bottle of rosé and a bag of deli snacks in the kitchen.

"There was an absolute disaster in the office last night!" Claire announces: "So I didn't get home until the early hours and had absolutely no time to think about what to wear!" Bianca smiles. "I wouldn't worry about that babe," she replies with a wink. "You're new to this, but I can tell you that if past experience is anything to go by, it won't be long until it all comes off again!" The women break into giggles and Claire gives a conciliatory shrug: "Better have a glass of wine in that case," she grins, and heads off in search of a corkscrew.

Tonight, it transpires, is something of an initiation for Claire, who, at 32, is the oldest of the four. For the first time, she will join the rest of her friends at what has become one of the most important nights in their social diary: a Killing Kittens party, an unusual private-members' event where, in the words of its organisers, "the sole aim is the pursuit of female sexual pleasure". According to Paloma, who is something of an old-timer (this will be her fifth party), if the event is a success, after an hour or two of cocktails and outrageous flirting, the gathering will turn into an orgy.

It won't take long to find out. But before we do, there is some serious preening, plucking and wine-drinking to be done, and while the girls prepare excitedly for the night ahead, they are eager to explain why attractive young career women such as themselves choose to spend their Saturday nights seeking out no-holds-barred group sex. "I know that for some people the idea is hard to grasp," Paloma begins. "But for me it makes perfect sense."

At 28, Paloma has a successful career in the City, which often keeps her at her desk from 7am until 11pm; the toll on her social life can be heavy and means she has limited time to dedicate to forging a meaningful relationship. But not having a boyfriend, she insists, shouldn't mean having to forfeit her sex life: "I've always had a strong libido," she says, "and with a high-powered job to contend with, I find sex is an amazing way to unwind."

Not that she isn't interested in finding a partner in the long term. On the contrary. Getting married and having children is a priority. Paloma's own parents have been married for 29 years, and the secret of their relationship, she says, is that they both waited until they found the right person. "Many women get involved in unsatisfactory relationships just because they believe that any companionship is better than none. But that is not what I want for myself. I want to wait until I meet the right man, and when I do, I'll concentrate my efforts on making our relationship last." Until then, she says, she will continue to ensure that she still gets her thrills, whenever and however she wants. And it seems she's not alone.

Judging by the booming numbers attending parties such as tonight's – Killing Kittens alone boasts 6,500 members, 70 per cent of whom are women – we are witnessing something of a sexual revolution, or at the very least, a new age of female sexual assertiveness.

According to the Adult Industry Trade Association, a record number of sex toys were sold in the UK last year – the majority to young women. Of the £100m-worth of toys bought, the biggest grossers – by a long stretch – at outlets such as Ann Summers, Love Honey, Myla and sextoys.co.uk, were hand-held vibrators, designed for women to use on their own, or with their partner. Whereas a few years ago, the mere mention of a Rampant Rabbit would elicit uncomfortable squeals, today high-end vibrators – some made of gold and costing hundreds of pounds – are treated as ornamental treasures to be displayed and celebrated, not hidden away in a drawer.

One wonders what the physician who invented the "manipulator" in 1870 – a steam-powered dildo used by doctors to relieve symptoms of "hysteria" in women – would make of this trend. "I'd love to see his face," says Paloma. "If only he'd known what he was starting!" On a more serious note, Paloma says that the widespread availability and popularity of vibrators has helped to normalise discussions about the female libido. This, she believes, is very empowering. "Like when Kate Moss was spotted buying a vibrator in New York. It's only a small thing really, but it helps show men and women that it's OK to take charge of your sex life, and that helps other women realise they are normal for having a healthy sex drive."

Two bottles and much preening later, the now somewhat inebriated gaggle arrives at the allotted venue for tonight's party, a discreet private mansion in central London. It is 10pm and outside there is little sign of life, but behind an imposing front door, some 200 revellers are getting into the spirit of the occasion. Once inside, it becomes clear why the girls feel so at home here. Like them, 70 per cent of the clientele are female, well-educated and conventionally good looking.

The few men in attendance shuffle about, forming feeble packs – safety in numbers, one imagines, for this is a place where they constitute the submissive gender, and they know it. Men can't even attend these events without a female chaperone, and once there, they are forbidden from making any kind of advance. So until they get an invite to "come and play" from a "kitten", they have to stand on the sidelines and watch the action unfolding.

We arrive to a surprisingly tame scene. Downstairs, the champagne flows freely while revellers exchange polite, even wary, niceties. Some of the groups, quite clearly new to this, seem unwilling to commit to a conversation with their new acquaintances in case they are, according to some private code, verbally contracting themselves to the other group. But it appears they needn't have worried – for when the party gets going, it becomes a free-for-all. It takes a while to find this out, however.

For the first hour or two, you might even forget the reason why you're here. Apart from a couple of topless young women, who could be escapees from a frisky college party, the vibe is low-key – a bit of dancing, a little flirtatious banter, but not much more. And then, like something from a vampire movie, at 1am almost to the second, this subdued affair transforms surprisingly quickly into a full-blown orgy.

By the time I return to the main upstairs salon – in which a huge pile of adjoining mattresses, overlooked by a floor-to-ceiling mirror, is placed tellingly in the middle of the room – the walls are lined with couples and huddles of women, all looking on eagerly as some 20 bodies (male and female) get to it. There are threesomes, foursomes and indecipherable groupings in the throes of various intimate acts, some having full-blown sex, either oblivious to or spurred on by their audience. Sometimes it's hard to tell where one group stops and the next begins. More and more of those on the sidelines begin to slide into the pile, without any obvious invitation, and no one seems willing to turn down anyone who wants to get involved. (It's hard to tell how widely condoms are being used, but I did notice a basket of them by the front door.)

As I make my escape, I catch a conversation by the ground-floor stairway: a naked woman, recently returned from the bedroom, addresses an apprehensive-looking friend: "Why don't you just come upstairs?" she asks her reassuringly. "If you feel like getting involved, that's great, but if you don't, that's cool too." The pair join hands and move towards the stairs.

I later tell Paloma that the whole affair has left me feeling a little unsettled. But it shouldn't, she insists: "Whether or not you personally agree with what happened is sort of by-the-by. The point is that we acknowledge that a lot of women have the same desires as men and should have the chance to explore them."

It is the acceptance that women have needs and desires that lies at the heart of what Paloma insists is a new sexual outlook. If we continue to be shocked by women going to sex parties, or using vibrators, then we are doing the female population a disservice. "This is supposed to be a progressive, liberated society," she argues. "And yet some people still insist on making women feel like social pariahs if they admit to enjoying an orgasm. I think it's about time we faced up to the fact that some women just enjoy sex. Is that really so hard to believe?"

All names have been changed