"I feel like less than a woman," says 34-year-old Marian Jones. Since having her second child four years ago, Marian has lost all desire for sex. "I feel guilty every time I say no. I slink into bed after he's gone to sleep... I can't bear to be touched." So her marriage is a disaster?
"He's my soulmate," she says. "I just feel no sexual desire for him, or anyone else." She is is not alone. Recent surveys show that around 43% of women have a "sexual dysfunction". This doesn't mean we're all fantasising about dogs, dressing as nuns or doing odd things with the Hoover nozzle.
It means that nearly half of us are complaining of nightly headaches, or faking orgasms, or finding sex painful, or simply lying there, one eye on the clock, feeling nothing much at all. So why is it happening to so many women? The answer isn't straightforward.
The medical term is "female sexual dysfunction", a complex area roughly divided into four basic conditions: low sex drive or an aversion to sex; an inability to get aroused; an inability to orgasm; and pain during sex.
None of these sounds too unusual: you're unlikely to feel like love's young dream, say, after giving birth or consuming 16 tequila slammers. But these normal sexual hiccups become dysfunctional when they're recurrent, persistent and cause personal distress.
There can be obvious physical causes: the menopause or a hysterectomy can disrupt the production of the hormones oestrogen and testosterone, both key components in desire. Other factors, like heart disease, diabetes, childbirth and breastfeeding can mess up your sexual functions too. But in many cases for youthful, otherwise healthy people like Marian, the cause isn't clear.
The bad news is that, unlike 12m men worldwide, Marian can't just pop a pill, sit back and feel the heat rise. Last week Dr Rosemary Basson, associate professor of obstetrics and gynecology at the University of British Columbia in Vancouver, presented the findings of her study into whether Viagra works for women.
The short answer is "no" (at least, not usually). Basson focused on premenopausal women - average age 37 - who were experiencing FSD. Viagra worked about as well as a placebo pill, but the drug might help certain sub-groups (namely some menopausal women).
"We're continuing our research," says Heather Van Ness, a rep for Pfizer, which makes Viagra. "We feel this [area] is significantly more complicated than erectile dysfunction."
It's perhaps odd that Viagra won't work widely on women. Arousal in both sexes works in a similar way: we all have smooth muscle tissue in our genitals that engorges with blood when we're aroused. Viagra works by stimulating this blood supply. Researchers hoped the drug might relax the tissue in the clitoris, allow it to swell, and bingo: goodbye dysfunction; hello boys.
It didn't. So if fixing the blood supply to our clitorises won't make us sexy, what will? Dr Michael Crowe, a consultant psychiatrist and head of the couple and sexual clinic at London's Maudsley hospital believes the pharmaceutical solution is too simplistic: "Women's sexuality is more complicated and multidimensional than men's. The physical and emotional are connected in a way that makes them inseparable."
But it seems we're often being treated according to a male pattern of stimulation - arousal - satisfaction. This is certainly what Dr Laura Berman and her sister Jennifer (a urologist) believe is wrong with FSD treatments today. Together they run the women's sexual health clinic in Boston, where all patients are initially given both a physical and a psychological consultation.
"Women experience sexuality differently from men," says Laura Berman. "For women, sexuality involves many components - both emotional and physical - which happen simultaneously."
Our tendency to lie back and think of England doesn't help . "All too often, whatever the root cause," says Laura Berman, "women don't let their partners know how to satisfy them sexually. We pretend to be aroused, or to reach orgasm and then find ourselves either having to live with a permanently unhappy sex life, or let the other person know."
By this time, sex has usually become fraught or non-existent and the relationship is under severe strain. The realization of the scale of female dissatisfaction has galvanised drug companies. They've also noticed that a generation of "baby boomers" is about to hit the menopause - several million solvent women who believe in (and will pay for) their right to sexual pleasure.
Since Pfizer made more than $1bn in the US and more than £8m in Britain in the first year alone, the potential gain is massive. Dr Riley says: "It's a new field, but everyone is racing to catch up."
There are many different drugs in development, but research can be roughly divided into two areas: hormone treatments (usually oestrogen, testosterone, or a combination of both, administered by injection, pill, patch or cream) and "vasodilators" (drugs like Viagra, that dilate blood vessels and increase blood flow to the genitals).
This month the US food and drug administration approved a device optimistically named "Eros" - a vacuum pump that increases the blood supply to the clitoris. The next wave of treatments, says Dr Martin Cole, the recently retired head of the sex education institute in Birmingham, is based around the brain chemicals of seratonin, dopamine and drenaline.
"Once we fully understand the role of the brain chemicals in arousal and response," says Cole, "we'll have some amazing treatments." But maybe medication is missing the point. "Sometimes, people just do stop fancying each other," Cole suggests. "The best sex therapy under those circumstances might be to point out that sex is just one part of a relationship jigsaw, and it doesn't have to be the most important part." After all, if nearly half of us don't like sex, then FSD is almost normal, isn't it? A bit like cellulite or PMT?
"Absolutely not," says Laura Berman. "Addressing female sexual dysfunction is the final frontier of the women's movement." We need to "take responsibility for our sexual satisfaction" and "feel that we're entitled to it".
We can start by explaining to our partners exactly what, how, where, when we want it. This won't solve all problems - those rooted in a physical condition will need medication too - but taking control of your sex life has to be a good start.