As a Japanese saying puts it: Suezen kuwanu wa otoko no haji (It is shame for a man not to eat a feast placed before him).

In other words, any real Japanese man would never turn down a sexual offer from an attractive female — whether he’s married or not. Adultery and infidelity are time-honored Japanese traditions — for the nation’s men.

If a Japanese man wants an affair or a sexual experience outside of marriage with no strings attached, he has his choice of sexual massage parlors, “soaplands” (licensed brothels), escort services (aka “delivery health”), shady hostess clubs or numerous “meet-up” websites, which are often thinly veiled prostitution venues. If he’s well-connected and wealthy, he can even register with a members-only club and be matched up with a perfect mistress.

What’s more, it’s not uncommon for married men to join gōkon (group blind-date parties) — sometimes without even bothering to take off their wedding ring.

Unfairly enough, there really are few sexual services readily available for a woman seeking paid sexual gratification with no strings attached — or even easy ways to have affairs. There have been sexual massage services for ladies, and not too long ago there was a brief boom in take-out hosts. However, there is really nothing even close to the male-centered sex industry which accounts for 2 percent of Japan’s gross domestic product.

Things are tough for a married woman who just wants some honest hanky-panky, a simple one-night stand — or “true love.”

And Japanese wives do have affairs; they’re just more discreet.

According to the findings of a 2008 poll by the Ministry of Health, Labor and Welfare, more than 20 percent of married men (aged 16-49) had had extramarital sex within a year of the survey. For women, the equivalent figure was 11 percent.

So, Japan’s men aren’t the only ones who may have something on the side — though even that works to their advantage, too, as the penchant for hitozuma (sleeping with other people’s wives) is no rare thing. In fact as a form of schadenfreude, it’s succinctly expressed in the everyday saying, Hito no fukō wa mitsu no aji (The misfortune of others is a taste of honey).

The hitozuma services, which often claim all their sex workers are really married, have always been a popular genre in Japan’s “pink” industry. In January, prefectural police in Ibaraki busted a yakuza boss for running a popular prostitution service called Married Women Island. Of course, there was no data available on how many of the women were actually married. However, judging from past cases I covered as a reporter on the police beat for a vernacular national newspaper for several years, it’s usually only 20 percent — the rest are pretending to be married. Let the buyers beware.

One reason that numerous surveys suggest affairs may be trending upward is the well-documented fact that sexless marriages are on the rise and now account for more than half the total, according to a condom-maker’s annual survey.

Enter just-launched ashleymadison.jp, the Japanese branch of the world’s most successful dating site for married people. Yes, you read that correctly — after all, this Toronto-based operation that’s been around since 2001 has, as its slogan: “Life is short. Have an affair.” You can sign up if you’re single, but you will be introduced only to married men or women, depending on your preference.

The website clearly taps into a juicy Japanese market — those legions of couples in sexless marriages, who have been ignored up to now. In the first 10 days, more than 120,000 people joined — with women outnumbering men by 2 to 1.

In the 1990s in Japan, with the publication of a book titled “Shitsurakuen” (“Lost Paradise”), followed by an eponymous spinoff movie, extramarital affairs suddenly became trendy even though the book ends tragically. In contrast, the website tries to assure a happy ending — a gokurakuen.

The smooth-talking and married CEO of the parent company, Noel Biderman, believes the new, localized website will give Japanese women, married or in a relationship, the same opportunity for infidelity as men. He says, “The longer people live together, the more likely they are to lose sexual interest in each other. But there’s no sense in ruining a good marriage or putting kids through the pain of divorce. Having an affair — a discreet affair — may be the best thing for everyone.”

The site is set up to make sure the women using it have a distinct advantage — as they don’t have to pay to join or use it, whereas the men do. And with women in most families in Japan holding the purse strings, if they do embark on an affair, paying for love hotels and dinners should be easy.

Additionally, the site may also tap into a completely ignored market — the many gay and lesbian Japanese who are trapped in socially approved heterosexual marriages.

If adultery sites take off, sexual services for women may be next. After all, isn’t it time everyone had an equal opportunity for hanky-panky?

Investigative journalist Jake Adelstein is the author of “Tokyo Vice,” a board member of Polaris Project Japan and a contributor to The Atlantic Wire and japansubculture.com. His email address is jakeadelstein@me.com.