it seems like there is more to the story than what you tell.
http://www.wa-pedia.com/culture/sex_in_japan.shtmlSexless couples & extramarital affairs
Japan has a surprisingly high number of sexless married couples by Western standards. There is an explanation for this. Marriages arranged by the family or friends (such as o-miai) are not unusual in Japan. Couples rarely live together before marriage, and the Japanese never have children without getting married first (unless it's an accident), because the Japanese law still does not recognise fathers of children born outside wedlock. Divorce is still very widely frown upon. In other words, the law and one's family pushes people to get married, even if the family has to resort to an arranged marriage, but they won't let them divorce afterwards, even if things don't work out.
To top it all, Japanese culture makes a clear distinction between a childless woman and a mother. Mothers are not seen by their husbands as sexually desirable anymore. Once a child is born the mother becomes like a relative, not a sexual partner anymore. (This may explain why so many families only have one child).
The difference between most Western countries and Japan is that Westerners would divorce or abstain if they were caught in a sexless relationship, while Japanese stay married and have extramarital affairs. Besides, northern and western Europeans (at least) are increasingly having children without ever getting married. This was unthinkable up to the 1950's, but liberal laws in the '60's and '70's have changed all that. A report in the Economist showed that nearly 80% of Swedish couples with children were not married. Only 50% of English parents are. The highest rates of married parents were found in Mediterranean countries, with Italy reaching 75% - a far cry from Japan's 99.9%.
This state of affairs has implacable consequences on the way Japanese society functions. The Japanese sex industry is the biggest in the world (in terms of money generated). Porn magazines are available at every convenience store and read widely, even in public. Many Japanese men are obviously sexually frustrated. According to the Durex global sex survey the Japanese are the national group that have sex the least often among the surveyed countries around the world. 37% of Japanese respondents said they had sex at least once a week (as opposed to 78% in China, 70 to 87% in southern Europe and 55 to 80% in northern Europe), and a dismal 15% admitted to being sexually satisfied.
Extramarital affairs are an almost culturally accepted part of life in Japan. A middle-aged married man won't be in trouble at home for seeking romance with young bar hostesses, or having sex with prostitutes in a soap land or "massage parlour". These are accepted facets of married life in Japan.