Japanese people have some very mature ways of dealing with this subject which can be hard for some westerners to deal with.
Showing insecurity or "doubting" your partner will cause many Japanese people to lose respect for you, especially your partner!
The attitude is usually that people will cheat if they want to cheat and asking questions is not cool even if they are cheating!
The deal seems to be that we should show trust and confidence in them, no matter what.
This can be difficult for some westerners to deal with but, if we are honest with ourselves, it is a sensible and mature approach.
If our partner is thinking of cheating or tempted to, they are more likely to do so if we are being insecure, not trusting them, or otherwise behaving in an uncool way!!