In its cover story last month, The Economist newsmagazine looked at the issue of "Asia's lonely hearts: Why Asian women are rejecting marriage and what that means." It offered many reasons — including economics, education level, changes in family structures and gender roles, divorce difficulties, and demographics — for why many Asian women (and of course, by extension, Asian men) are marrying later or not at all.I commend The Economist's well-intentioned attempt at dealing with an important social issue. But its discussion left one major stone unturned: sex.At the risk of turning this month's scribbling into a Hugh Hefner column, I think it incumbent upon those of us planning a life in Japan to consider a fundamentally unhealthy social phenomenon: how sexuality in Japan is downplayed, if not encouraged to be omitted completely, from many married lives.First, an axiom: Healthy adults have sex throughout their lives, and this should not necessarily change just because people get married.However, in Japan it often does.A "sexless marriage," according to the Japan Society of Sexual Sciences, is generally defined as one where couples have sex less than once a month.Sumie Kawakami, in her book "Goodbye Madame Butterfly: Sex, Marriage, and the Modern Japanese Woman," cites a 2006 joint survey by the Ministry of Health, Labor, and Welfare and the Japan Family Planning Association that found more than a third (34.6 percent) of all Japanese married couples could be classified as "sexless."This is a rise from earlier surveys and should be discussed in Japan as a social problem. After all, Japan has a falling population and a birthrate at the bottom of the world's scales — demographic trends that garner more than their fair share of media attention.But sexlessness is hardly seen as problematic in Japan. Quite the opposite. Hark back to the 1990s, when the sekkusuresu state was portrayed in the media positively, even as a natural outcome of marriage.There is of course plenty of coupling and intimacy before matrimony (as I'm sure many of our readers can attest), but once kids are in the picture (people are even discouraged from having intercourse while pregnant), sex can decrease markedly or even become nonexistent for a habit-forming period of time.Ask why and the reasons are usually forthcoming: One side is often "too tired," "It's a nuisance," or the kids are sleeping in the same bed, etc. The more cynical cite the cruel aphorism, "You don't need bait for a fish already caught".But there is a fundamental difference here from attitudes in other developed societies, where sex even into old age ("orgasms at sixty!" on supermarket shelves, and don't forget Oprah, Dr. Phil, etc.) is seen regardless of family lifestyles as a healthy and essential part of a relationship.Not in Japan, oddly in this "must try harder" society.Then this discouraging set of expectations gets recycled back into our media and becomes self-perpetuating. Group-think gets people off the hook from trying to maintain intimacy, while people made to feel they "want sex too much" are sometimes told to take their loins elsewhere. No wonder sleeping around in Japan is a national pastime.One might say this is just an outcome of modern life in a crowded society. But similar modern pressures and overcrowding exist in other countries.Consider a more worldwide sampling of the issue.In 2005, Durex, the world's largest condom maker, conducted a Global Sex Survey (see www.durex.com/en-jp/sexualwellbeingsurvey/documents/gss2005result.pdf) involving 317,000 respondents in 41 countries. The survey found that Japanese had the least sex in the world, at 45 times a year — far less than second-from-bottom Singapore (73 times a year), and even farther from the world average (103 times a year, meaning twice a week).Moreover, less than a quarter (24 percent) of Japanese surveyed said they were "happy" with their sex lives, significantly lower than the global average of 44 percent.Durex's more recent Sexual Wellbeing Survey, involving 26,000 interviews from 26 countries, found Japan at the bottom again with even lower results (15 percent satisfied).One might counter that everyone exaggerates or is reticent about their sexuality, skewing the stats. But in international comparisons, Japanese are rarely shy about presenting an upbeat image of their society to the world. Such low figures for Japan say to me that people are being brutally honest about sex, or that a lack of sexuality is not perceived as something negative.This matters. It is one more disincentive to marry in Japan. Indeed, why lock yourself into a marriage to someone who becomes a sibling instead of a spouse?Sex life is not part of the dialog on the decline in Asian marriage. But in Japan's case, it should be.It is Japan's worst-kept secret.Debito Arudou's novel "In Appropriate" is now on sale (www.debito.org/inappropriate.html) Just Be Cause appears on the first Community Page of the month. Twitter arudoudebito. Send comments to firstname.lastname@example.org. Responses to last month's column, "The loneliness of the long-distance foreigner," will be published and posted online next Tuesday.