Roppongi is an area of Tokyo well-known for its foreigner-friendly nightlife. It's especially notorious as a hunting ground for young Japanese females looking to secure western boyfriends. Since it's frequented by off-duty military personnel and foreign businesspeople, it's ripe territory for such gentle "predators".
My husband has never availed himself of the nightlife there, of course. During his year as a single male in Japan, he was devoted to me at a distance and we were married when he returned for his second, much longer stint. One of his students had a discussion with him about Roppongi and he told her that he had no interest in going there and in fact had never been to any of the nightclubs or prowling spots because of its reputation for easy one-night stands and gaijin-hunting women.
The student recommended that those points were the very reason that he should go there in her opinion. She felt that he should avail himself of the opportunities while he could. He reminded her that he was married and even if he had the chance to cheat, he would not. In fact, he told her that he had no interest whatsoever in casual sex and he didn't have any interest in it even before he was married or even in a committed relationship with someone. The student's response was to laugh and say, "you're a liar."
Her take on men was such that she couldn't fathom any male who wouldn't want to sleep with any woman who freely offered herself and disbelieved that he wouldn't consider cheating on his wife if he had the chance. I'm pretty sure this was her personal low opinion of men who are ruled by their gonads rather than their hearts but the situation in Japan in regards to cheating spouses appears quite different than that in the west. This student felt that 90% of husbands cheated on their wives.
Since this particular student was so jaded, my husband asked others how they felt and they said they felt it was more like 60%-70% cheated on their wives. The main problem with this question when asked in "scientific" surveys is that it's virtually impossible to get accurate statistics so I can't say how his students' viewpoints measure up to the reality. One of the reasons for this is that some Japanese men feel sex they pay for doesn't constitute infidelity so they are likely to report that they do not cheat if they go to prostitutes. The other reason is that asking the question as part of a survey is almost useless when the respondents may choose to lie.
So, I can't speak to the reality of how many men cheat on their wives but I can say that I've encountered more than one foreign man who had a Japanese girlfriend who told him it was not a problem if he cheated as long as she didn't find out about it. Marriage is generally viewed somewhat differently in Japan than in the West. I've met a few people who were very devoted to their spouses but the overwhelming majority tend to view marriage as a partnership where each party fulfills his or her role to make a successful family and home. Many people feel that the whole notion of romantic love burning bright throughout a marriage is unrealistic.
Given my views on marriage (which include the fact that you can be completely mad about each other forever), this doesn't exactly strike me as "right" but it does go a long way toward explaining the high divorce rate in the United States and the lower one in Japan. Of course, the lower rate in Japan is perhaps more greatly explained by the economically disadvantageous position of women in the culture.
However, if you expect every day to be a date after you're married or if you expect to be in love forever and that fades away in an American relationship, in many cases you end up divorced. In Japan, if you don't expect that sort of passion in the long run but you're both doing your "job" (as it were), you stay together. It's always been my impression that the Japanese set the bar "lower" when it comes to the romance part of marriage and "higher" when it comes to each party fulfilling their mutually-agreed upon roles. That is, a deadbeat husband who doesn't work or make enough money to support his wife and kids would be more likely to cause his wife to divorce him than a good breadwinner who cheats on his wife.
Still, this western mind, particularly this romantic one, bristles at the idea of casual infidelity and it's a much harder push to avoid being ethnocentric about this topic than some others.