The more they represent, the more they have authorities. If they are not of good moral standards, it is like entrusting a cat with fish. There may be corruption made by the representative. Thus, the higher moral standing and being beyond reproach are severely required for them. It is for sure.
If someone representing your country is not beyond reproach and of good moral standing then how can they gain the respect of those who they are representing on your behalf. How can they be trusted to make the right moral decisions or act in away that does not reflect badly on your country? If they are not clean living then they will make a poor representative!
Someone representing a government in international affairs should have demonstrated good moral standards, at least with respect to a uniform understanding of morality. He or she should practice behavior that genuinely makes the world a better place to live, and he/she should endorse views that promote the betterment of the country and the world.