Even if there is statistically more cheating in Japan, I think the issue is more that they, and us as well, have a problematic view on romance and relationships. The fact that he's so afraid just to ask, like it's the most devastating thing in the world, tells me that he put too much of himself and his selfworth into this relationship. If more of his selfworth and happiness came from other sources than this one person, it would be much easier for him to confront them and risk losing them in a fight.