'Sex is really important. It's the glue in any relationship, and if you've been together for a long time you've got to make more effort to keep it interesting.''

It's the sort of remark that not so long ago would almost certainly have been made by a man. But, in fact, it was spoken by Rebecca, a thirtysomething married woman with two children -- and it is further proof that in today's world, women are calling the sexual shots as much as men.

She speaks from experience. Five years ago, she became so fed up with her husband's lack of interest in her that she decided to do something practical about it and take a lover.

"My husband stopped fancying me when I had our first child,'' she said. "We used to have a great sexual relationship, but now he keeps making excuses. I don't want him to see me just as a mother to his children. I found myself in a situation I'd never envisaged, but I decided it was better to have an affair than to separate.''

Men, too, don't want to suffer in silence.

"My wife and I have a great marriage and I love my three kids,'' explained John, 49, "but I don't have anywhere near as much sex as I'd like. My wife is always too tired, and when I bring up the subject, it starts a row. There must be lots of other men like me, who pretend to the world that everything's OK but who are far from happy.''

The happiness is relative. As well as the obvious health hazards, what you gain from an affair is small compared to what you risk losing -- except perhaps if an affair is used as a deliberate exit strategy from the marriage.

It also only takes a second's carelessness for your infidelity to be discovered. In the five months I spent researching infidelity and interviewing middle-class adulterers, I came across countless stories where one of the partners, usually the woman, finds an incriminating text on her partner's mobile. The straying partner often explains that he was only having sex with someone else because he loves you and is trying to keep the marriage together -- but such comments are rarely reassuring.

The two things in life that we fear the most are betrayal and humiliation. Or so says Cathy Breslin, a counsellor, hypnotherapist and life coach from Kildare.

"When your husband has gone off with someone else you get both. Betrayal is the most difficult thing to come to terms with; and it's harder for a woman to hide her feelings. Women harbour that pain; it can take them a long time to get back on track."

And it hits hardest when women are in their forties.

"Especially in Ireland," she says. "The mentality, here, is still for women to help build up her husband's business and his dreams. It's, 'I have done all this for you so that everybody is going to do well from it.' When a husband breaks that contract it is really hard to get life together again."

So what should the wronged partner do (if indeed there really is a wronged partner)? Affairs are often the result of something wrong in the marriage, rather than just the cause.

A consensual view is to do nothing in a hurry. Both women and men who rage and shout and tell their loved one to pack their bags and leave often live to regret it.

Jill, a 42-year-old accountant and mother of three, is one. She split from her husband two years ago because he had a fling and now thinks she acted precipitately.

"I threw him out, despite his plea that he loved me,'' she said. "I really miss him. I thought we were happy and my pride was wounded, but I should have waited until I had cooled down a little. I have been very lonely and we have begun to meet again to see if there is anything left between us. I don't know what will happen.''

Gerry Hickey, a psychotherapist and counsellor, says that Irish couples tend to separate after an affair.

"Even a no-strings-attached affair is seen as an invasion of the relationship," he says. "There is too much hurt involved; particularly for women.

"Men often can't take the betrayal either," he says, "though for them it's about being possessive. They get very angry and jealous. Their masculinity is threatened.

"Whereas for women, it is the love which is at stake; it's the stability of the family, and the welfare of the children that are threatened."

If the wronged party can hang on, science is on their side. Infatuation has a chemical base as well as an emotional one. When two people's eyes meet across a crowded room, their pulses quicken and their hearts race. As they are drawn irrevocably together, certain changes will take place in the body's chemistry. Three chemicals -- phenylethylamine, dopamine and noradrenaline -- are released and together produce that amazing euphoria of passionate love.

The chemical combination can last in the body for between six months and two years. As levels drop, it is replaced by oxytocin, the so-called "cuddle hormone'' which induces nesting behaviour. Most affairs end around this time.

Cathy Breslin believes that couples should stay together if at all possible.

"They should learn how to communicate and they should get help. They should keep remembering that time when they fell in love and things were going great for them," she says.

"For the most part, today, people are so tired in the evenings that they become distracted from their relationship. They don't relate to each other except at weekends.

"When I get them to remember when they were in love, though, in today's 'have it now' society, they are likely to reply, 'but that was then'."

"My wife didn't understand how much sex meant to me until she found out that I had had a brief fling,'' said 48-year-old Edward, who works for a marketing company. "It took months of conversation, but she gradually began taking more care of herself than she had done, lost weight and showed more interest in sex.''

Mary, who has had two affairs during her 18-year marriage, is more hard-headed and thinks that both sexes have to move with the times.

"I don't believe that having an affair should lead to divorce,'' she insists. "The whole structure and nature of society has changed. Any couple who have been together for any length of time should try to understand the circumstances they find themselves in and give their best shot at working it out.''

‘Whether couples stay together depends on the level of communication’

According to Christine May, a counsellor with MRCS Counselling, couples often fail to recognise the natural dips in a sexual relationship; and the male/female differences.

"When women have children, for example, it is natural that they have less interest in sex.

"They need emotional closeness to stimulate their sexual interest. This can be a difficult time, particularly if they are working. There can be terrible pressure on them.

"Whether couples stay together after an affair will depend on the foundation of the relationship and the ability of each partner to move on and forget," she says.

"It depends on their level of communication."

‘It is profound unhappiness that drives people into the arms of someone else’

David Miller, who runs a discreet dating agency for married couples, believes that adultery -- from which he earns his living -- is deeply misunderstood.

"No one walks down the aisle thinking: 'Now I'm married, I can be an adulterer'. People go into marriage thinking that it's a lifetime commitment and most of them are deeply upset when it goes wrong.

"If a woman doesn't want it to happen to her, she shouldn't shrink away from talking about sex and then giving her man what he wants and needs.

"From my experience, it is profound unhappiness that drives most men and women into the arms of someone else. Many women have thought about it for years before they are unfaithful. And although men don't think about it for as long before they act, many feel quite isolated.

"They take their partner's rejection of sex as a rejection of them as people. Many come to see me more in sorrow than in anger."

- Additional reporting by Sue Leonard