Nancy dearly loves her husband.

She cannot imagine life without the man, a company director five years older than her - but she simply falls for new men who pay attention to her.

Each of her love affairs last a short time before she, wrecked with guilt, would end it, praying that her husband would never find out. There were close calls, but she has explained her way out of trouble.

Nancy, who works in the media industry, has sworn time and time again that it would be the "last one".

She has lost count of the number of lovers she's had. Yes, she's with one now.

Svelte, with long flowing hair, Nancy insists she's no seductress. Dressed modestly in her executive clothes, Nancy doesn't search for prey in watering holes.

Each of her lovers has been a man she meets on the job. They are successful, confident and often, married.

And they go after her first, she says.

Sexual equality has hit a perverse note here, according to a recent Durex survey. About one in five men cheat on their partner.

The same number of women fall prey to cheating. Says Nancy: "I have intense feelings for my lover...I think I thrive on them thinking that I am great, desirable and sexy... The attention also makes me feel loved."

Her husband is a faithful and religious man. His work keeps him busy.

For her, it's her lover who keeps her occupied. She's always on her phone, checking e-mails and messages. And she just can't quit.

It took some persuading for her to talk. Having a string of lovers is not a source of pride for her.

Nancy's story demonstrates another of the condom manufacturer's findings. Men tend to have casual sex on the sly, while women take on lovers.

Brian, 40, is just like Nancy. And like her, Brian told this paper he simply cannot stop philandering. We are not using their real names to protect their identities.

When he was in his 20s, Brian sought sexual adventures and jaunts behind his girlfriends' backs.

He also believed that visiting prostitutes was just a phase that would end once he got married.

But now, five years after his wedding in 2006, he continues to seek prostitutes once every two months.

For him, it is more physical.

Says Brian, an accountant: "The only thing that has changed slightly from my early years is my frequency (of seeing prostitutes).

"With a 5-year-old son to look after and my busy work schedule, there are only a few excuses I can make (to my wife) whenever I need to slip out of my home."

He knows what he is doing is morally wrong. Getting caught with the fresh scent of shower gel or perfume on his body would be disastrous.

So he smokes to mask any scent that could betray his rendezvous.

Brian dreads the thought of his wife, a housewife in her 30s, finding out about his two-timing ways.

She could ask for a divorce and get sole custody of their only child, says Brian.

Yet recognising right from wrong has never been an issue for Brian, who has slept with at least 20 women since his marriage.

He adds: "To me, it (sex with a prostitute) is an emotionless transaction and a way to de-stress.

"I move from one girl to the next like you would when buying maybe a toothbrush. When you're done, you pay for a new one.

"Despite my cheating, I still love my wife. My family comes first."

Well, maybe not all the time.

During one Chinese New Year eve, Brian felt the urge to have sex, but not with his wife.

"I managed to have one 'session' before rushing back for the reunion dinner," Brian reveals.

As he was a little late, his family had waited for him before starting dinner.

"I just can't stop and ignore what my body is telling me...I need the release."

Psychologists and psychiatrists say people like Brian and Nancy continue to cheat for various reasons.

Some do it out of sexual frustration with their current partners, finding their own sex life boring.

For others, deep unresolved emotional issues, like bonding difficulties, become a factor.

Some think that sleeping around is widely accepted by society, says Mr Daniel Koh, a psychologist from Insights Mind Centre.

"This can be due to wanting to improve one's self-worth by looking for ladies or men who find them attractive and desirable," he says.

"Or it may be a matter of collecting a number of 'trophies' to see how far they can go before getting caught (by their partners).

The news that both men and women (20 per cent of them) cheat is hardly surprising, says psychiatrist Tommy Tan.

Cheating is an innate quality in humans, he says.

Dr Tan, from Novena Psychiatry Clinic, says: "It's nothing unusual. Even birds, who are said to be paired for life, fool around in the animal kingdom.

"It's their way of seeking the best genes for their offspring."

What is unusual for Dr Tan are the cases involving a different kind of spousal infidelity.

In some cases he has seen, cheating spouses - both men and women - did not seek help because of their inability to be faithful.

Instead, they came to Dr Tan because they suffer from depression when their boyfriends or mistresses threaten to leave them.

Adds Dr Tan: "What's important about the sex survey is the proportion of men and women who are unfaithful.

"If the figures show a big rise and if the survey is representative of our population, then there is cause for concern."

This article was first published in The New Paper.