The Challenge of Cultural Relativism

A problem was brought forth in The Challenge of Cultural Relativism saying that this theory cannot be applied to certain things such as the earth being flat or spherical. If one culture believe the earth was flat but we believe that it is spherical, can we actually overlook that and say that there is both right and wrong in these beliefs? We now for a fact that the earth is spherical. I don't think this theory is meant to be applied to situations such as the previous, but moreover moral situations such as shown with the Greeks and the Callations.  Every culture has a different belief system which has different moral traits. Yes, there may be one truth out there that is the only thing that is right, but there is no way of knowing what that could be, so there is faith. Each culture puts their faith in their beliefs and to them, they are right because they believe that their truth is the only truth. In Things Fall Apart, many people would see the cultures marital practices as wrong, but according to their culture, they would see ours as wrong.  Right now even, there is the question of gay marriage. So many people claim it is wrong based on their belief system, but others have their own set of moral guidelines that allow such a thing. People need to be more open minded to see the difference in everyone's ideas and beliefs. So many problems have arisen throughout history because of this. I'm sure that the Greeks and the Callations despised each other only because of their burial rituals. They probably fought wars over it, just as there have been religious wars in the Middle East for centuries. If everyone were to apply this theory of understanding and tolerance towards other religions and cultures, we would not have so many problems in the world. Of course you can't force the theory onto scientific fact, such as the world being round, but there should still be an understanding that each culture can believe what it wants. We cannot condemn other beliefs or cultures merely because they are different.
Now, I'm not saying that you can just let terrible acts of inhumanity go because a certain group of people believes in something.  Take the Nazis for example.  Most people would agree that what they did was terrible and wrong, but the Nazis thought they were doing a great thing. Here is where the Theory gets blurry. I think that the solution is to say that groups and cultures that believe one thing should not be forcing their beliefs on others, and if they do, we should be allowed to stop them. Christians have always been forcing their religion on others for years and years as have other religions. This is wrong, even though they do it in the name of "God."  The people being repressed should have the right to push them away and refuse their beliefs.  Again with the current situation of gay marriage, many say that people are trying to force homosexuality into society, but in fact, it has been here for ages. They are not forcing their beliefs on anyone else, they are not forcing  others to be gay, so society has no right to fight them off. I'm not entirely certain if any of this made sense or if I was rambling, but hopefully, I got my point across.