It was in the early 1990s, when the British feminist Sheila Jeffreys migrated to Australia, that she realised just how huge the global sex industry had become. Prostitution had been legalised in Australia in 1984 and, looking through the newspaper one day, she realised with horror that women were being advertised for sale alongside their photographs. In line with the government, many Australian feminists had come to view prostitution simply as a form of work, and Jeffreys found herself appalled by this "neo-liberalism" - the complete lack of moral outrage about the buying and selling of women's bodies.
This was to prove one of the many spurs to writing her new book, The Industrial Vagina: The Political Economy of the Global Sex Trade. As professor in political science at the University of Melbourne and a long-term activist against sexual violence, Jeffreys' books include Anticlimax, which laid bare the myth of the 1960s sexual revolution, and Beauty and Misogyny, which uncovered the brutalities of the beauty industry. She tells me that The Industrial Vagina came about when she realised that the sex industry was "spiralling out of control".
Jeffreys' book explores how the sex trade has been transformed from a small-scale, furtive, vilified industry, into a hugely profitable and legitimised one. As such, the text teems with startling figures. There's the fact that the stripping industry is worth at least $75bn worldwide, bringing in $15bn in the US alone - more than the revenues generated by baseball. Or the detail that the pornography business is now so successful and mainstream that it is covered by the financial newspapers, with some companies, such as Beate Uhse - Germany's most successful sex-shop chain - listed on the stock exchange. "Prostitution is now a significant market sector within national economies," she says, with her usual forthright intensity. "The sex industry in the Netherlands, where [prostitution] is legalised, is worth 5% of the GDP."
This has been happening at a time when women's power has supposedly been increasing. Why would prostitution, stripping and pornography become more popular as women take a stronger public role? Jeffreys tells me that because women are now "able to say 'no' more often to degrading sexual practices", the use of prostituted women is a way for men to uphold their traditional privileges.
Jeffreys has also discovered just who is making money out of this market. It is, of course, a huge earner for pimps, traffickers and brothel owners, but many others are profiting too: hoteliers make extra revenue by supplying prostitutes to businessmen; taxi drivers make commission by delivering male tourists to particular brothels. "Billions upon billions of dollars are made off the backs of women in this industry," says Jeffreys, "and feminists opposing it are up against powerful groups of men, and often entire governments . . . Where is the criticism from the left of this gross capitalist industry? We can slate the tobacco and nuclear industries, but not the sex industry, in which the poorest and most disenfranchised women are abused."
The women involved rarely make money from prostitution, she claims, despite the popular view that the work is highly lucrative for them. Jeffreys includes evidence from studies carried out in Australia and Canada, which shows that the average annual wage for women in brothel prostitution is just £15,000. "We need to take this discussion away from the feminist arguments about whether it is harmful to women or not," she says, "and look at it as a massive, massive industry where the profits are not going to the women."
Jeffreys has never been afraid to address controversial topics, a trait that has gained her many detractors, as well as admirers. In 1979 she was one of the authors of Love Your Enemy, a booklet that questioned just how committed heterosexual feminists were to the women's movement. She has also criticised lesbians for using pornography, and has claimed that sex reassignment surgery is a form of self-mutilation. She says that she would love to write a history of the women's movement from a "revolutionary feminist perspective"; the common theme of her work is her firm belief that men maintain power over women by the act of sexual intercourse, and that heterosexuality is therefore bad for women.
This belief is reflected in The Industrial Vagina in her description of marriage as a type of prostitution; a legal transaction that has traditionally guaranteed sexual access to women's bodies in return for subsistence. "Prostitution and marriage have always been related," says Jeffreys. "What is shocking is that today marriage is becoming more fashionable amongst some young women". She writes that even in the case of employed, well-paid professional women "the right of men to women's bodies for sexual use has not gone but remains an assumption at the basis of heterosexual relationships".
She uncovers other, more glaring links between marriage and prostitution - such as the growing issue of mail-order brides. The mail-order bride industry is essentially a form of trafficking, she maintains, with many of the women who are advertised on marriage agency websites later being sold into prostitution by their husbands. The majority of men who access these services are white and from wealthy countries, and their brides come from cultures where the female stereotype is subservient and docile.
The strength of Jeffreys' new work lies in just how many aspects of the sex industry she covers, and her understanding of their intersections. For instance, she points out the links between mail-order bride sites and pornography; between lap-dancing clubs and trafficking operations.
She believes that the use of prostituted women by the military, which has long taken place around US bases in Thailand, South Korea and the Philippines, has been the most important factor in the globalisation of the sex industry. In countries "where the industry thrived under military occupations," says Jeffreys, "and the Netherlands where brothel prostitution is legalised, men flock from elsewhere to gain access to prostitution services." Jeffreys asserts that countries that have legalised prostitution are "pimp states". "If the state facilitates the prostitution of women, it is obviously maintaining male supremacy. These states are directly colluding in maintaining women's inequality."
As we speak, Jeffreys is preparing for a trip to Europe where she will be lecturing on prostitution as a harmful cultural practice, on a par with female genital mutilation. I ask if she is expecting criticism from her audience. "Of course," she laughs, "feminists always get flack when we tell the truth about the sex industry. But women will continue to be harmed by it so long as we continue to believe the lies."
• The Industrial Vagina: The Political Economy of the Global Sex Trade by Sheila Jeffreys is published by Routledge at £14.99, and can be ordered at www.routledge.com or through Bookpoint on 01235 400524.