Without feminism, gay rights would not exist. Britain's homosexuals would be imprisoned, married, or dead. But there is a homophobic lie so often repeated by feminists and female columnists that it has become an accepted truism: that the west's obsession with skinniness, with size zero, and the crisis of self-image that follows, is the fault of gays. Gay fashion designers want emaciated models, it is said, because they want women to look more boyish. They want women to look like young men – devoid of curves.

This was a line rehearsed in the Observer: "It has long been said that fashion is a con-trick by largely gay male designers to make women look more like men: breastless, hipless, as skinny as a boy." ("Vogue is not a magazine for children").

And in the Daily Mail, a similar line was peddled earlier this year: "The elite of mostly gay designers has been creating catwalk designs for pre-pubescent teenagers, and each year wanting models who looked less and less like women… The designers were wanting women to look more and more like young men."

This theory has never been challenged, becoming the sole socially acceptable line in homophobic thought. But it is not acceptable. It is simply bile hawked up from the darkest cavity of feminism's lungs. Why then is the argument such pernicious nonsense?

Implicit in the belief that gay designers want women to look more boyish is the notion that gay men are only capable of finding beauty in the masculine; that by making women more androgynous they become more alluring; gay men are aroused by boyishness rather than manliness. These are all untrue.

If you think gay men only have an aesthetic appreciation of either the androgynous or the male, you need only turn to art created by gay men – Michelangelo's Pietà, or Caravaggio's The Penitent Magdalene. Or you could note the adoration of womanly women in contemporary gay men's musical tastes – Beyoncé, anyone?

No matter how masculine – or boyish, as the rhetoric would have it – a woman looked, a gay designer would never find her sexually attractive. So why, we must ask, would they purposefully mould women in the alleged fashion? What would be the point? In short, gay men are attracted to men, not boys. To blur this line is to perpetuate the most poisonous of homophobic slurs: that gay men are all paedophiles.

And so we come to the real reason for fashion slashing women's curves: commerce. This isn't so much a patriarchal conspiracy as a capitalist one. The way to extract money from someone is to encourage fear and the desire to control. Fear causes us to buy that which prevents social rejection, to be ahead of the Joneses – deodorant, the latest cut of jeans, the constantly upgrading gadgets. And controlling tendencies lure us to products that shape ourselves and our environment: the Ajax, the Ambi Pur plug-ins, the liposuction. A thin control freak is fashion's ultimate customer.

Which brings us back to clothes. The history of fashion contradicts those who repeat the gay designer slur. Coco Chanel introduced the more boyish look – the structured, straight-up-and-down silhouette, ignoring the hourglass. It was women who made trousers – the archetypal male attire – fashionable: Marlene Dietrich and Katherine Hepburn. And it was a heterosexual male designer, André Courrèges, who first brought long trousers to womenswear in the early 1960s.

Today, which designer is most routinely feted for his celebration of the womanly shape? Roland Mouret, the gay creator of the Galaxy dress.

When journalists speak of an industry dominated by gay men they ignore both the plethora of female designers (Stella McCartney, Sarah Burton, Vivienne Westwood, Donatella Versace, Amanda Wakeley, Luella Bartley and more) and the commercially huge straight men such as Ralph Lauren and Tommy Hilfiger. They ignore too the immense power of predominantly female fashion editors. Gaultier and Lagerfeld don't control fashion; Anna Wintour does.

This anti-gay theory, this splutter of lazy, baseless thought, born out of justifiable despair at body fascism, deserves to die now. Or perhaps, more fittingly, it simply deserves to go out of style.

Patrick Strudwick won the Journalist of the Year at the Stonewall Awards and the Best National Newspaper Feature at the Guild of Health Writers awards in 2010 for his investigation into therapists who claim to "cure" homosexuality