Cheating isn’t necessarily more ‘acceptable’ in Japan (or other Asian countries, speaking as half Vietnamese myself), it’s just more forgiving. Like, you can’t go to a pub with work mates, who may know you’re married, and brag about your latest sexual encounters (though we don’t do this anyway even in committed relationships or marriages), so you still keep the cheating hidden. You’re actually liable to lose serious social standing and possible career opportunities if you are found to be cheating.
Marriages are much more like social contracts in Asian countries, where you marry to have a stable life, children, your parents approval, etc. as opposed to marrying for love like in western countries (though people do marry for love). But even marriages for love tend to be ‘forgiving’ of cheating though it depends on why they’re cheating.
A person who cheats for physical pleasure is more easily forgiven or overlooked, whereas a person who has feelings for whoever they’re cheating with isn’t. (Incidentally, men are more likely to cheat to blow off steam, but women who cheat are more or less emotionally checking out of their relationships).