I don't have this problem with my wife. She is an emotionally well-balanced and secure person. However the subject of mia nois came up a couple of times and she stated quite clearly that if I ever had sex with another woman, it was "goodbye".
And I have clearly explained that not that many Western men cheat on their wives.....at least not as many as in Thailand. So we both understand each other.
Have you sat down with her and explained at length, clearly and unequivocally, that you are satisfied with her and don't intend to cheat ? Because brief assurances are not enough for many Thai women, methinks.