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Two-timing politicians, take note: cheating has never been easier. AshleyMadison.com, a personals site designed to facilitate extramarital affairs, now boasts slick iPhone and Blackberry versions that help married horndogs find like-minded cheaters within minutes. The new tools are aimed at tech-savvy adulterers wary of leaving tracks on work or home computers. Because the apps are loaded up from phones' browsers, they leave no electronic trail that suspicious spouses can trace.

Even as public outrage boils up over the infidelity of South Carolina Governor Mark Sanford and Nevada Senator John Ensign, millions of Americans are sneaking online to do some surreptitious cheating of their own.

Unlike Craigslist's plain-Jane listings, AshleyMadison lets cheaters customize profiles, chat anonymously and trade messages about adulterous preferences — all in an effort to make cheating as simple as using Match.com.

The formula is working. AshleyMadison's membership has doubled over the past year to 4 million. The Toronto-based site, which takes its name from the two most popular female names in 2001, the year it launched, enjoyed another big boost this week, following Father's Day, when CEO Noel Biderman says men often feel underappreciated. Traffic to the site tripled on Monday. (Biderman says there's a similar boost in interest from neglected wives and girlfriends after Valentine's Day.)

Over the past month alone, 679,000 men and women have used the service to contact a cheating partner. According to their profiles, 92% of males on the site are married or otherwise attached, as are 60% of female members. No word on how many politicians have signed on.

Critics call AshleyMadison a cruel sex site that profits from marital pain. "This is a business built on the back of broken hearts, ruined marriages and damaged families," says Trish McDermott, a dating-industry consultant who helped found Match.com and Engage.com. "It's in the business of rebranding infidelity," she says, "making it not only monetizable, but adding a modicum of normalcy to it. AshleyMadison is making bad choices, broken promises and faithlessness look like something that's trendy and hip and fun to talk about at a cocktail party."

"We're just a platform," responds Biderman. "No website or 30-second ad is going to convince anyone to cheat," he says. "People cheat because their lives aren't working for them." Not everyone buys that line of defense. The Las Vegas Review-Journal recently refused to run an AshleyMadison ad referencing the Ensign scandal. But other racy TV, billboard and radio ads have succeeded in raising the site's profile over the past year to the point where by some measures it's in the top tier of dating sites, with tens of millions of dollars in annual profits. AshleyMadison charges members $49 for a package of credits they can use to contact up to 20 members. Members don't pay to receive messages, just to initiate contact, so many women end up using the site for free.

Maybe that's why many of the site's new members are female. Biderman says the proportion of women on the site has grown from 15% — when the service quietly launched in 2001 — to nearly 30% today.

Dorothy, a 45-year-old Floridian whose screen name begins with SexyMom, says she's been married for 20 years but started using the site four months ago because her husband constantly turned down sex and refused marriage counseling. "It's like the seven-year itch, but 20 years later," she says. "My husband never throws me a compliment. Now I meet guys who say, 'You're so hot,' or 'You have great eyes.'"

On a recent weekday, 38 men sent messages to Dorothy, who checks these e-mails on her phone during breaks at work. "If I wanted to schedule something for morning, noon and night, I could," she says. She ignores most inquiries, especially those from immature 20-somethings or older men seeking a one-night stand. "I'm looking for a friend, possibly with benefits," she says, "but I'm not out there to shake someone's hand and open my legs."

So far Dorothy has met seven men through the site, she says, including a wealthy, 49-year-old divorced doctor with whom she hit it off. Dorothy says her husband would be livid if he found out, but he doesn't know how to use a computer. "Now I don't have to bug him for intimacy," she says.

AshleyMadison isn't the only site aimed at under-the-radar relationships. Sites like EstablishedMen.com and SeekingArrangement.com all offer variations on the theme. But AshleyMadison is the most successful site openly capitalizing on extramarital affairs.

And for that, Biderman offers no apologies. "Humans aren't meant to be monogamous," he says. So would this free-thinking CEO mind if his own wife used his site? "I would be devastated," he says.

• Jeremy Caplan
• AshleyMadison.com, a personals site aimed at facilitating extramarital affairs, now has mobile apps