The night sex came back into fashion began with an invitation in a black leather envelope to a catwalk show in a former monastery. It was a sultry evening in Paris last September, and the sun was setting behind the iconic YSL initials, picked out in neon, as the new designer at the storied house of Yves Saint Laurent, Anthony Vaccarello, unveiled his debut.

The look was Robert Palmer's Addicted To Love video meets young-Kardashian on a date night, spiked with a twist of Helmut Newton. Long legs in high heels, oversized earrings grazing bare oiled shoulders, boned velvet corsets and sheer black lace. And then - just in case the message wasn't getting across - Binx Walton strode the catwalk resplendent in a black leather mini dress, cutaway to reveal one glittery silver nipple pasty.

It may have been the moment sex appeal made its official Paris Fashion Week comeback, but the signs had been there a while. Two weeks earlier in New York, Jeremy Scott's show was a love letter to sleazy Eighties Manhattan nightlife, all latex trench coats and T-shirts printed 'Rated X'. There were bikini-clad pin ups on the shirts at Alexander Wang, and 'Hustler' logo polo shits at Hood By Air. And in Paris, the scent of sex stayed in the air all week, from the catwalk corsets at Olivier Theyskens, to the latex 'condom cape' at Balenciaga.

How do nipple pasties and thigh-high boots - with all their associations of porn and stripper-wear - fit with fashion's newfound feminist consciousness?

Hold up. What's going on here? After all, we're talking about the very same Paris Fashion Week where, just three days after the nipple pasties, Dior's first ever Creative Director, Maria Grazia Chiuri celebrated her landmark moment with slogan T-shirts proclaiming "We Should All Be Feminists." The Paris Fashion Week where, in contrast to the thigh-high boots on the catwalk, many in the front row were comfortable in simple white trainers and classic Gucci loafers. How do nipple pasties and thigh-high boots - with all their associations of porn and stripper-wear - fit with fashion's newfound feminist consciousness? Is fashion having a sexual awakening or an identity crisis?

Antonio Berardi, whose dresses are loved by Gwyneth Paltrow, Blake Lively and a league of loyal clients for their killer combo of knockout sex appeal and silky sophistication, believes that seduction will never go out of style. "Never. Sex and clothing fundamentally go hand in hand. Every man and woman dresses to impress, whether it be the other sex, same sex or both," he says. From American Apparel to Calvin Klein, from Sophie Dahl for Opium fragrance to Gucci's infamous logoed pubis, the briefest history of fashion advertising confirms sex as a fashion perennial - as does the view of Natalie Kingham, buying director of Matches. "If we ever see sexy clothes, we do buy into them," she says. Sex sells.

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But what looks sexy right now has a distinctive contemporary flavour. Think of the trend, begun on the Prada catwalk back in February and recently championed by an off-duty Gigi Hadid, for wearing a corset over a T-shirt. It is a look that subverts the traditional sexuality of the corset, so that it looks "almost like armour", as Selfridges womenswear buying manager Jeannie Lee puts it. "Sexy now is very strong."

"Sexy today is about confidence", says Stuart Weitzman, the godfather of the over the knee boot. "Sleek, chic, versatile," is how he characterises the look; sex appeal is almost incidental. For those of us who travel for the collections, the weeks immediately after each month of shows are a time of style recalibration.

After the high-octane vibrations of fashion week and the street style circus, I find that this is when the shifts in silhouette and mood percolate through into what I will wear to the office, or on a regular Saturday. Tellingly, one of my first post-shows purchases this autumn was a pair of Stuart Weitzman plush grey suede over the knee boots. Call it the spirit of Saint Laurent, but after checking out these boots on other women for months, I came home from Paris Fashion Week emboldened to buy a pair. And yes - to answer everyone's first question - I wear them to work.

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"To me, the image of the season is the velvet corset and jeans with a YSL heeled shoe, from Vaccarello's first collection," says Natalie Kingham. "I keep seeing it in my mind's eye. Something about the silhouette and the message sums up the woman we call our warrior woman, who wants to look powerful and sexy. The rise of Balmain, of shoulders and corsets and thigh-high boots, but also the aesthetic of Givenchy, Alexander McQueen, Versace and Altuzarra all appeal to this customer."

You might assume that fashion is simply reflecting a culture more sexualised than ever - but Sarah Shotton, creative director of Agent Provocateur, believes the opposite is true. "The problem is that we are having too little sex, not too much. That's why fashion is obsessing over it now. It's a fantasy, because in real life we don't have time for sex any more. Ten years ago you could go home and be intimate, but now we go home and stare at our phones all evening. Fashion expresses our fantasies, as much as our real lives."

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Technology is never far away from any aspect of how we live now. One of the iconic 'looks' of the modern age, which has had a measurable impact on fashion, involves no clothes at all: the nude selfie. Which seems counterintuitive, until you consider that not even the most-liked catwalk photo of any given season could hope to reach a fraction of the audience that have seen famous nude selfies by Kim Kardashian and Emily Ratajkowski.

According to e-commerce site Lyst, in the 48 hours after Kim Kardashian posted her infamous 'When you're like I've got nothing to wear Lol' bathroom selfie, wearing only two black censorship bars, searches for 'black bandeau' bikini-style tops were up 406%, as shoppers attempted to recreate the look. A black bandeau style bikini by Lisa Marie Fernandez received 82,000 page views in just three days. In a visual world where the nude selfie rules, fashion is taking its cues from stylish images in which clothes barely feature.

Kim Kardashian maintains that nude selfies represent female empowerment and liberation. Others would argue that they perpetuate the commodification of women's sexuality for commercial gain, and send a message to young women that being sexual revolves around how you look to others, rather than how you yourself experience sex. The jury remains out on that one, but the debate has brought the notion of female sexual empowerment back into the spotlight.

Jess Morris is the designer of Rockins, the cult brand which began with silk scarves and now makes the sexiest, most apple-bottomed jeans in Britain. Her personal aesthetic, honed over two decades as a fashion PR and by dint of being on the guest list of any party worth being at, is classic rock'n'roll sex appeal: spike heels, tight jeans, black eyeliner, Mica Arganaraz-esque shaggy curls. "Fashion helps us identify each new cultural stage," she says. "It can be a reflection of oppression, either politically or sexually - or of freedom." Rockins celebrates a freedom which she sees in young women around her, "who can now express their equality through wearing gender blending clothes and a band T, or cut off denim shorts and fishnets. It's about expressing yourself and being heard."

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The modern take on hotness comes in many guises. "The last time we saw a real resurgence of sexy was with the popularity of the rebranded Herve Leger bandage dress in the early 2000s" says Jeannie Lee. "Then there was the Preen power dress. The impact of that style and shape was massive. It reminded women of the power of the dress - and of the power of sex." Fashion is always in thrall to the laws of action and reaction, and that era was "a fresh antidote to the new bohemia which was popular at the time. By contrast, sexy feels much more multi-faceted now. You have the classics, like Alaia corsets or the peekaboo sheers at Lanvin, but you also have the new style boudoir pyjama dressing - decadent, sensual and tactile."

In other words, the new sexy is relevant to your wardrobe, even if you are not in the market for thigh-high boots or corsets. "Being sexy nowadays is as much about what you're not revealing, as what you are," says Net-A-Porter's Sarah Rutson. "It's no longer about showing your cleavage, or being overt in body hugging mini dresses. Women can be sexy wearing a deconstructed dress with cut out details revealing their shoulder and collar bone. We're seeing the reworked cotton shirt worn off the shoulder or tied at the waist to reveal a hint of skin, as seen at J. Crew and Jacquemus. I also love Ulla Johnson's reveal and conceal dresses, which feel both flirty and feminine and show off tiny slivers of midriffs and shoulders. In pretty prints or simple cottons these feel like a very modern approach to what is the new sexy."

That sexy dressing has moved beyond showing skin is particularly pertinent at this time of year. Even the most dedicated Saint Laurent fan isn't going to be be busting out those miniscule dresses anytime soon, unless she wants to risk hypothermia. The Net-A-Porter customer, however, isn't going to let a cold snap come between her and hotness. "Right now, she's using knitwear to be sexy," says Sarah Rutson. "Look at Tibi's one shoulder cut out knitted sweater and Dion Lee's open back jumper. Sexy knitwear would once have been seen as an oxymoron, but it's big business now." And then, of course, there are shoes: Balenciaga's white pointed stiletto boots and Attico's satin shoes with ankle cuffs can both bring fetish attitude to a winter wardrobe.

"If we ever see sexy clothes, we do buy into them."

The new sexy defies neat pigeonholing. Its messages are most defiantly mixed - and this, perhaps, is precisely what gives it a fashionable spin. Judith Clark is curator of The Vulgar: Fashion Redefined, a recent exhibition at the Barbican Art Centre which explores what vulgarity means, and how this has changed, with pieces from Pam Hogg, Vivienne Westwood and Manolo Blank on display. "The thing that most people think of when they think of the word "vulgar" is about someone not knowing what to show, and what to conceal," says Judith. "It's about what is fitting, what is appropriate. 'Sexy' is usually the description given to something that conveys a seductive message explicitly, making the intention very clear, while the vulgar is perhaps associated with not knowing where those boundaries lie." In fashion, with its dramatic tide tables, the tectonic plates of those boundaries are liable to undergo volatile shifts.

Whether or not nipple pasties are to your taste, fashion would be a duller place without sex. "When it comes to taste, I am one for flavours in fashion," says Antonio Berardi. "That doesn't necessarily mean an excess of salt, but a hint of spice and seasoning well used. Even the most conservative fashion has the potential to be sexy. A button is there to be un-buttoned, a zip to be un-zipped and an imagination to run rife with possibilities."