This approach to Ethics is called moral relativism . Moral theories are either absolutist or relativist .
Moral values are relative to a culture and time.
Moral Absolutism says that there is a single eternal moral standard. Relativists believe that moral values are relative to the different cultures in which they are instantiated, which means that it is not fair to judge the behavior of one culture using another culture's moral code. When cultures collide, we need to be tolerant. In order to be tolerant we have to know what makes up a cultural group. It is possible to have a cultural group of one individual - and we would not be able to judge them using 'our' standards.