Growing numbers of married people are turning to internet chat rooms for sexual thrills, a US study has found.
Most spouses who got involved with the opposite sex over the internet did not think they were doing anything wrong, said the report by a University of Florida researcher.
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But partners felt betrayed by the virtual infidelity, even though in most cases no physical contact had taken place.
"The internet will soon become the most common form of infidelity, if it isn't already," said Beatriz Mileham, from the University of Florida, who carried out the new study.
"Never before has the dating world been so handy for married men and women looking for a fling.
"With cyber sex, there is no longer any need for secret trips to obscure motels. An online liaison may even take place in the same room with one's spouse."
Chat rooms are the fastest rising cause of relationship breakdowns, according to counselling groups in the US.
The problem would get worse as the number of people online continued to rise, said Ms Mileham.
For the study, the researcher interviewed men and women who used chat rooms specifically aimed at married couples.
She found that most people said they loved their partner. But the anonymity of the net provided an outlet for those seeking an erotic encounter.
"All I have to do is turn on my computer and I have thousands of women to choose from," said one of the men questioned for the study. "It can't get any easier than that."
Most people ventured into the chat rooms because of boredom, a partner's lack of sexual interest, or a desire for variety and fun.
"The number one complaint from men was lack of sex in the marriage," said Ms Mileham. "Many of them said their wife was so involved in child-rearing that she wasn't interested in having sex."
From virtual to real
The study found that what often started out as just friendly chat turned into something much more serious.
Almost a third of people taking part in the study went on to meet the person with whom they had made contact.
All but two ended up having a real-life affair. In one case, a man had 13 affairs with women he had met over the internet.
"We are hearing from therapists around the country reporting online sexual activity to be a major cause of marital problems," said Al Cooper, author of the book, Sex And The Internet: A Guidebook For Clinicians.
"We need to better understand the contributing factors if we are going to be able to warn people about the slippery slope that starts with online flirting and too often ends in divorce."
For the study, Ms Mileham interviewed with 76 men and 10 women, aged between 25 and 66, who used Yahoo's Married And Flirting or Microsoft's Married But Flirting internet chat rooms.