"Racist"- what does that word mean to you? Is it someone who judges people by their race (for better or for worse), or is it someone who thinks their race is superior to all others? I've heard people argue that we're all racist in some way because we have preconceived notions about how we think certain groups should act. I've heard someone say that racism is being proud of whatever color you happen to be. They're all similar ideas, but different enough to cause confusion.
Most people agree that racism is bad, and most people I know would claim not to be racist. I think that someone who claims not to be racist probably isn't one, because I'm among those that defines racism as the belief that one's own race is superior. Most people are self-aware enough to know if that's what they believe.
If somebody has preconceptions about other races, some people would call that racist, regardless of whether or not they think themselves superior. I don't know which definition is right (it's likely they both are), but I do know that saying "we're all racist," is bound to annoy a lot of people. I think the point they're trying to get across is that we all judge, but calling everyone racist isn't the right way. People are going to get defensive, and rightly so. They deny being racist because their definition of the word differs from the one who accused them. When people argue, especially about things of such a sensitive nature, they can become confused about what the other person is arguing about. They may both agree that everyone has preconceived ideas about others, but if each of them has a different definition of the word, then they'll never understand each other.
"Racism", "Patriotism", "Socialism"- one word is never enough to explain such complicated ideas. If you are of the opinion that everyone who judges people is racist, then try applying the same idea to words. Everyone has their own ideas of what it means to be black, brown, or white, and everyone has different ideas about what words mean; judging either too quickly can be dangerous. The next time you argue, make sure you know what you're arguing about.
PS: If you were wondering whether or not I'm racist- I'm not. But if you think everyone is, then maybe I am.