I had this guy that I worked with in a Japanese company horn in on this Japanese girl I was occasionally sleeping with. I'm not sure what if anything he was telling her, but my fun ended soon after that.
And the dude was a white American guy.
I don't think that being covetous or attempts to sabotage relationships or cheating on your mate are uniquely Japanese phenomena. It's a different society, so these things manifest differently, but this whole "the duplicitous Japanese" stereotype is straight out of WWII propaganda.
Look, sorry not to always give a glowing report about anything and everything related to Japan. As if in Japan, everything and everyone is perfect Nirvana... Sorry to spoil the fantasy... I think trying to project and push such a fantasy is much more of what propaganda is. And must we attempt to bully everyone who doesn't agree with the Japanese Nirvana fantasy or politically correct narrative?
I'm speaking to what my experiences and those of people that I know. As to the reason how or why it might be culturally different in this particular regard:
1) There isn't as much awareness or fear of violence for possible passing certain relationship boundaries.
In the U.S. and many Western cultures, you know that "scheming" (which depends on one's view) or talking to somebody's woman behind their back can result in violence or even death. It's more understood.
My perception is that a lot of Japanese guys might not see it that way or understand the possible ramifications of their actions. They see a personal opportunity for themselves, and selfishly or blindly go for it.
2) Lack of diversity and jealousy
It occurs even in the U.S. and other countries where guys get jealousy fits over interracial or international relationships. If a woman of his color or nationality is dating somebody different, certain types of guys have strong emotional and mental issues.
Especially when a guy feels he is "superior" to another guy or perceives a woman choosing a guy outside of his group is somehow an insult to him (as if he is entitled to all women of his color or ethnic group).
But where such type of guys with racist or xenophobic views tend to be more physically overt in other countries, Japanese with such views can be more covert and the behavior more outwardly ambiguous or oddly passive-aggressive.
3) Seeing things as "His success, is my failure." "His failure, is my joy." "Hito no fukou wa mitsu no aji."
Some guys see themselves more strongly in terms of being in competition with other guys over women or in general. This competitiveness can manifest itself more strongly with Japanese guys that you know versus you don't know or because you are a foreigner. "How did he get with X woman, when I couldn't?"
Of course any race or nationality of guy can get all bent up over petty jealousy or competitiveness, but this type of behavior is well known in Japanese culture. Cultures can also have different levels of it that manifest to higher degrees in personal relationships.
"Hito no fukou wa mitsu no aji." It means, "Another man's misfortune tastes like honey." And many Japanese know exactly what this is about.
This past Friday was on a date with a gorgeous Japanese woman. We were in a bar that I know, with booths and leather sofas. As we were hugging and kissing up on each other, it was very clear we were there as a couple. 1 older guy gets so infatuated with her, he comes tripping over himself and makes the request she dance with him. She didn't want to, so told him sorry. Older dude then gets upset and starts trying to harass my girl into dancing with him. She seeking to avoid a confrontation and to humor the old guy, spontaneously jumps up and gives him a little dance for a few minutes. Instead of enjoying that moment, the old Japanese guy then proceeds to persist even more into making himself bothersome. "This your girl?" "Where did you meet her?" etc... To the extent that the bar staff had to tell the old guy to leave.
Now you might be saying, that's just a random event that can happen anywhere... About 1 hour later, younger single Japanese guy in his 30s enters the bar. Sees us dancing and grinding, with my girl, he clearly knows what's up. However, becomes infatuated with my girl too, so decides to just awkwardly and continuously stare at her. Dude even unbelievably gets a chair, directly in front of our booth. Unfortunately the bar seats are arranged this way, but people usually know not to be so mindlessly rude. Dude continues staring us down until the point of ridiculousness. I have to get up and tell the guy to move. Another kind of odd scene.
I've lived in Japan for MANY years, and can tell a long list of stories of the "odd behavior" of Japanese guys (who I both knew and didn't) that just so happen to occur involving women or on dates. Now for guys that are not dating a lot or are often staying home, they may never notice, but other guys that are out a lot can notice.
Do realize that there are different experiences and views about Japan. There are many reasons for this, but we all travel different paths. Also, Japanese culture isn't going to be the same as anywhere else, so clearly there will be differences or perceived differences, so nothing wrong with acknowledging this.