If your spouse seems oddly devoted to an outdated cellphone, beware: Japan's philanderers have discovered that one particular flip phone, the Fujitsu F-Series phone, is the perfect choice for those who keep a little something on the side.

The F-series phone, introduced in 2002, has a unique privacy setting that's virtually impregnable, according to the Wall Street Journal. Once a contact is designated as private, the phone hides all their incoming emails, calls and text messages. The only clue that a private contact is calling is a barely perceptible change in the phone's battery symbol or antennae bars.

To one Japanese blogger nicknamed "Bakanabe," who boasts about trysts with multiple girlfriends, his Fujitsu phone is irreplaceable. "Women may want to check my phone for strange emails or calls when I'm not around. With Fujitsu's 'privacy mode,' they can't see that information at all," he told the Journal.

Bakanabe has some advice for would-be cheaters: "The key is to give off the impression that you're not locking your phone at all." The phone has even earned a nickname of its own: "uwaki keitai," or "infidelity phone."

For good or ill, cellphones have proven to be an indispensable part of modern sexual relationships — even for those too young to marry. A 2012 study found that 28 percent of Texas high-school students had sent nude pictures of themselves via cellphones, and more than half (57 percent) had been asked to send a sexually explicit photo to someone else. The study also revealed that, as common as those requests are, they often irritate those who have been asked.

And rates of sexting, or sending sexually explicit text or images via cellphone, have skyrocketed among U.S. college students, 78 percent of whom said they have received sexually suggestive text messages, according to a 2011 study. Two-thirds of college students admit they have engaged in sexting, usually within a relationship.

Japan, however, seems to be the dominant player at the intersection of sex and technology. Virtual boyfriends and girlfriends are wildly popular among Japanese men and women — one young Japanese man even hosted a real-world, live-cast commitment ceremony with his virtual girlfriend.

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