As Madonna and Guy Ritchie's marriage fails because of an alleged sex drought, our expert says that they are not alone - sexless relationships are a modern phenomenon

Before recent showbusiness reports you'd never have dreamt that the phrase “sexless marriage” might apply to Madonna and Guy Ritchie. After all, Madonna's numerous reinventions have invariably depended on sexual themes ranging from the kinky fetish girl to the restrained English rose ready to be unfettered. And the tall, handsome Ritchie exudes rugged sexuality.

But last weekend a newspaper alleged that an 18-month sexual drought - and Madonna's apparent gym-addiction - caused their relationship to crumble so spectacularly.

“He got more and more frustrated as she spent nearly half the day working out. Afterwards she'd be too tired to make love,” a source told a Sunday newspaper. “Towards the end Guy wouldn't be there. He'd get fed up waiting for her and go down the pub with his mates.”

The level of sexual activity in a relationship is a good barometer of its health; at least where both partners consider themselves “sexual” people - as opposed to the minority of couples who simply aren't bothered by impulses that drive others between the sheets.

I'd guess both Madonna and Ritchie are in the former camp. But for couples with, say, a volatile mix of high-achieving personalities, careers keeping them apart, gruelling workout schedules, parenting, step-parenting and adoption issues, it's obvious how tensions can mount. It's no surprise that they are less likely to want sexual intimacy with the person they've come to view as the enemy within.

In my 20-year experience as a psychologist, life coach and sex expert, I have found that people within a relationship invariably use sexual activity - and their sexual desire for their partner - as a way of measuring how much they care for each other. The bedroom becomes a litmus test, be it a battleground or an unspoken arena of seething resentment.

And with our increasingly frenetic modern lifestyles - our obsessions such as exercise, long working hours, material success and lots of unhealthy socialising - opportunities for resentments to fester abound. At some level our sexual feelings are intact (which is why we get so hung-up, hurt, and bitter over a lack of sex) and yet we have no sexual energy left for our partners.

For my new book, Sizzling Sex, I spent three years gathering information from more than 400 people and what I found was astounding: 90 per cent won't have tried anything new sexually - not even the tiniest little thing - since about their first anniversary. Plus, once they are past the two-year mark, they are unlikely to ever try anything new.

I've come to call this the “two-year turn-off” because many partners start to find their uninspiring sex life does just that. There's often little motivation to have anything more than infrequent sex, which can cause relationships to flounder.

Annie, a 36-year-old ward sister, had gone through the golden-honeymoon phase with her partner Peter, 39, and slipped back into the old social life - the nights out with her colleagues that she had loved before her relationship became serious. After two years, Annie was happy with occasional, not-too-demanding sex.

Peter, a teacher, became increasingly frustrated. He wanted to occasionally experiment in bed and didn't feel that was too much to expect. He wondered why his once “naughty nurse”, ready to slip into her day uniform for some night-time excitement, had disappeared.

Also, as he was going through a bad time at work, he longed for the comfort that good sex with Annie used to provide. On reaching crisis point, they found a compromise that satisfied both their needs: she went out with colleagues no more than twice a week and he tempted her with a few new tricks in bed.

As their experience shows, sex can be hugely important to a relationship in a number of ways. It can be exciting as well as sustain people during bad times. It sets a relationship apart from something that's purely platonic and can give a sense of security and intimacy. Importantly, it makes you feel desirable and good about yourself.

Mark, 43, a research scientist, is satisfied with exactly the same sexual routine every four or five weeks. All his extra energy goes into evening classes leaving little for his wife of five years, 45-year-old Philippa. She confided in me as a friend that after the first couple of years together their sex life became lacklustre. Philippa admitted that they hadn't tried anything new for a long time.

From my interviews, I told her frankly that she is in good - not necessarily comfortable - company. For many couples, there seems to be a progression from the honeymoon phase, where we let parts of our life - such as regular gym workouts and excessive socialising - fall to one side so that we can spend loads of time in bed with our new partner.

Then this sexually charged period calms down and people start picking up the aspects of their life that fell away. According to my interviews with countless clients, so many couples are unable to achieve a satisfactory balance between these two poles.

So why only this brief window of sexual activity and then the inevitable decline? Recent research into the biochemistry of attraction has found that our ancient ancestors were procreating like mad during the first six to 12 months after meeting. They couldn't keep their hot (and probably hairy) hands off each other. Usually a pregnancy would occur and sexual activity would drop off for a period of child rearing.

That biochemical backdrop is still present in our sexual relationships today. But we are more evolved than our ancestors socially and know full well that we have control over and above our biology and that we have the freedom to have sexual relationships that are equally about continuing pleasure, not just procreation.

With so many couples in this situation the question surely is: just how important is sex for a relationship? It is hugely important because of the ripple effects when sexual initiative goes out of the window. This is always complicated by people being resistant to talking about feelings that make them vulnerable. On top of this, partners have different expectations and it's easier to retreat into your own life rather than explore these.

If, though, you want to be in a satisfying, happy and sexually charged relationship then you must give of your time, energy, and goodwill. Even if you don't feel like sex on a particular evening, you should let your partner know how much you love him or her and that you're happy to talk about things rather than ignore them.

It takes only a little effort to freshen up your sex life, but most people, who are pursuing so much outside the relationship, feel that it's all too much and do nothing to keep it reasonably interesting. They know what's happening, but they're so tired from juggling the many aspects of their lives that they don't know where to start to change things.

I've written my new book with this in mind so that people can dip into it occasionally and select a little sex tip to try. The premise being that you don't have to swing from the proverbial chandeliers to liven things up.

Considering the pace of modern life, it's understandable that many break-ups have their origins in a frenetic outside routine and a boring sexual routine. If that happens to you then you and your partner have only yourselves to blame.