Like most of the men I met, Craig wasn't simply seeking a bang against the bedpost, but the excitement of a romance. His sense of guilt was evident, but his drive to relieve himself of the sadness of his home life was greater. "I would like to feel excited about seeing someone again. It's better doing it this way than meeting someone at work who might want more, and then it gets all complicated," he told me.
Another man, whom I met in his lunch hour in a boutique cafe in London, was even more direct. "This is me seeking my old identity," he said. "You compromise in a marriage. You lose a lot of your free time and choices. When I met my wife, for four years we were infatuated. It's amazing, but it isn't reality. It doesn't stay like that. It develops into something else, but it will never be the same. Sometimes I feel oppressed and I want to go back to what I was."
I met some men undercover. Others knew I was a writer and agreed to open up. Graham was one of these. A retired health-care provider, he showed no sign of slowing down at 67. He'd used so-called adult dating sites for years, with his wife's blessing.
"A marriage becomes more like friendship," he said.
"After 30 years, it doesn't matter who it is, you don't fancy each other over that time. My wife's not interested in sex - she's gone through the menopause and it went after that. She is into the grandkids and holidays.
"She knows I use these sites, but I don't talk about it. If I'm going to meet someone, I just say I'm going out. But I never, ever lie. I give my wife anything she wants, any clothes she wants. I'm lucky she understands. The trouble with most women is that they don't want to do it themselves, so they can't accept their men doing it."
The hackers of AshleyMadison.com, known as the Impact Team, are demanding the site goes offline or it will release users' information. It's unlikely that a threat from a little-known group will make a difference, though. Affairs have prevailed across all civilised societies for millennia, from royal mistresses to Japanese geisha, Chinese concubines and the harems of the Ottoman Empire. Many high-profile figures such as Bill Clinton and Tiger Woods have risked all in the name of lust.
Ashley Madison has some 37 million users across 30 countries. Meanwhile, adultery is cited as the reason for divorce in one in six cases in Britain, according to the Office for National Statistics. Clearly there is an appetite for it, which would exist whether or not sites like these were there to facilitate it. What such sites do is allow affairs to take place in a controlled environment, where participants are aware of the relationship's boundaries.
That's how Noel Biderman, the Canadian founder, defended his site when we spoke earlier this year. "Monogamy is a construct. It was born hundreds of years ago for hereditary reasons - it was never about love," he said. "People all around the world seek encounters to get away from that. We provide a conventional channel for people to do so. People never used to live that long. Now they live to their eighties. That's a long time to spend with someone."
The effects can be devastating nevertheless. Jo Welch is co-founder of the websites Woman-scorned.co.uk and Kids-scorned.co.uk, support groups for those affected by infidelity. "Affairs get found out, we see it all the time," she says. "If someone has an affair, the unity of the family disintegrates. Whether the couple stay together or not, kids pick up on a fraught atmosphere and it is unsettling.
"For women - or men, if the wife has cheated - they suffer with confidence issues. If you've dedicated 20-odd years to your home life, it is devastating when that is blown apart. Some people say affairs are universal, but heartbreak is universal, too."
In the UK there are few things that evoke disapproval like an illicit affair, but other cultures take a more relaxed approach. In France there is the "cinq a sept" - the two-hour window dedicated to clandestine encounters. In Japan, so-called "love hotels" hire out rooms by the hour. Keys are purchased through a vending machine, so guilty lovers can indulge themselves discreetly.
Every man I spoke to said he still loved his spouse, but the relationship had become one of domesticity. Such meetings often do involve deceit, but in my experience they did not involve malice. These husbands did not want their marriage to end - they just wanted to push at its obvious limitations.
Without this outlet, I suspect some would have parted ways with their wives many years ago.
