I think there is just simply 30+ years of perception on white foreigners and how we behave. Some merit of truth to it, as well, for what its worth. Though, I always make the point that, if we are talking about those in a relationship, the chances of cheating are the same whether you are TWese, American, Chinese, Japanese, Brit, man, woman, etc.
If somebody wants to cheat, they will cheat, and it isn't limited to a nationality. Many TW'ese take 'business trips' just to the other side of the straights in order to have 'good discussions and meetings with happy stories.'
All of that said, I think it wise to appreciate where it (insecurity, distrust, perception, whatever you choose to call it) comes from. People are just people at the end of the day. If you grow up in an environment where this is what you see your whole life, then it becomes engrained (and particularly for the TW'ese case, it is abbetted by the general lack of trust after being screwed over by everybody from the Japanese, Chinese, Americans, the global community, and their own government). All plays its part, really.
I think once you know where it comes from, the best course is to just be patient and continue to show them through actions. So far, so good.