What is un-American?

What is un-American? Is it difficult to define? Who gets to decide what un-American is?
Being un-American is simply not having pride in this "fine". You might not agree with everything that has happened or agree with the laws but American is a beautiful place even if the surface is cracked. But that's not a valid argument, is it?. America is not a place with one culture or one religion so how can it be defined as one? Or be judged as one? Being un-American is not being able to accept that American is the melting pot of the world. Everything is invited but not everything is accepted. People that are different and people that are similar are brought together in one place called America. You can accept it or you can deny it. There's no one that can decide what's un-American, it's just an opinion, educated or not it's just an opinion.