FORTUNE — Last spring, while French president François Hollande was in the throes of his secret love affair, a Pew Research Center poll found that a mere 47% of his fellow citizens consider adultery to be morally unacceptable. Even if the French are blasé about fooling around, though, that doesn’t mean they like getting caught. Mobile devices and the Internet (and paparazzi armed with both) make it easier to know when a spouse is behaving badly, but technology can also help cheaters cover their tracks. A quick Google search reveals that providing alibis for philanderers has become a veritable cottage industry in France.
An old-school private detective named Gérard (whose last name is withheld for the purposes of this story) started what he claims is the country’s first alibi service in 2005 after realizing during his investigations that “unfaithful people need room to be free.” Now he plays both sides, tracking straying spouses as well as providing excuses for them through his site, mon.alibi.free.fr. He claims he’s never had a conflict of interest.
Adulterous clients ask him for a range of fake documents, from plane tickets to programs for (imaginary) conventions. His secretary creates the alibis on her computer, and for an extra fee will place a phony phone call or send an SMS to back up the subterfuge. Gérard can also suggest entire scenarios for clients who are short of ideas.
A bogus invitation for, say, a weekend job training session will cost 39 euros. Each additional alibi — an e-mail confirmation, a restaurant receipt from the town where the person is supposed to be — runs 39 euros more. “It can add up quickly,” Gérard says. And though infidelity might be recession-proof, it seems that hiding it is not. “A few years ago, clients would easily drop 500 euros on alibis,” he notes. “Now when we tell them each document will cost 39 euros, they say oh là là — but that’s the price for a roll in the sack.”
In 10 years he has provided alibis for more than 2,000 people. Surprisingly, he says that 70% of his clients are women, and that they use his service differently from men. “Men come to me afterwards, looking to explain where they were the weekend before. Women plan. They’ll say, ‘In two weeks I’m spending three days with my lover, and you have to find me a good excuse.’”
Since Gérard started, many others have entered the marketplace, needing little more than a computer and some graphic design skills to get started. A 32-year-old named Sébastien, who lives near Clermont-Ferrand, started his online service (expert-alibi.com) after seeing the concept on TV six years ago. “I was unemployed and needed to work, so I thought, why not this?” he recalls. He set up his site at the same time he was studying to be an at-home childcare provider, and now he divides his time between the two — taking care of kids until 6 p.m., then forging alibis into the night. His clientele is slightly more male than female, and his revenue fluctuates from about 800 euros a month in the winter to twice that during the summer, when the French take long vacations from work and, evidently, from marital life.
Sébastien is married with four children but feels no guilt about the service he provides, saying there might be fewer divorces if people got busted less often. He adds, “Before alibi agencies came along, people who wanted to cheat figured out ways to do it, generally by asking their friends to cover up for them. We’ve brought a bit more professionalism to the process.”
A less hands-on option exists for two-timers on a budget: A French smartphone app called SOS Alibi, which has been downloaded 120,000 times since it came on the market in 2011. Users can program the app so that a computer-generated text message or phone call arrives on their mobile phone at a requested date and time, seemingly from a friend or colleague suddenly calling them away.
The service is free, offered as a complement to an extramarital dating website, Entre-Infidèles.com (both owned by a company, Neteden, based in Aix-en-Provence). Damien Poulain, Neteden’s chief operating officer, says the site’s growth has been exponential, with 600,000 subscribers since it went live three years ago, and he claims it’s the third-biggest on the global market after Ashley Madison and Gleeden (also started by two Frenchmen). Poulain also believes these websites don’t fan any flames that wouldn’t already be burning. “Is there any more infidelity since they appeared?” he asks. “I don’t think so. Internet is there to offer services to people who need them. We don’t create behavior.”
MORE: Fast-growing dating apps
Neteden is planning some changes to improve its adultery-friendly products: camouflaging SOS Alibi’s icon to appear more innocuous, modifying the website name to something less obvious. On the site, they are adding a calendar where subscribers can note the exact days and hours they will be available. And they will include suggestions for activities that people can do together, such as cooking classes or a museum visit. “Extramarital sites overlook the whole notion of flirting,” Poulain says. “It’s not because two people are unfaithful that you can just put them face to face and they’ll jump into bed together. You need to help them get to know each other, to build up desire.”
As each new concept takes hold, technology-assisted infidelity has become an arms race of sorts. While Monsieur is surreptitiously ordering his next alibi, Madame can surf for ways to test his trustworthiness on sites such as Tester-sa-fidélité. A Frenchwoman named Shana started the service in 2011 and now hires 40 part-timers to hit on suspicious clients’ partners by phone, Internet, or in person. For 290 euros, she will send a “tempter” or “temptress” into the path of an overly flirtatious spouse to strike up a conversation, make gentle advances, and see if the suspect takes the bait.
How many fail? “A lot,” Shana says. “The people who use our services generally have good reasons to doubt their partners.” But she’s quick to point out that this isn’t entrapment, as the seducers are never vulgar and don’t propose sex. “It’s just a way to get some answers,” she explains. “Often we find out that people we’ve lived with for years — we don’t know them all that well after all.”