Kids who grew up in an environment where anonymous interaction is the norm (such as kids who have anonymous social chats at school, or do in-game chats etc) have grown so accustomed to it that I don't think they even see it as sexist, racist, bullying etc. Most would probably never say any of this to someones' face, not out of fear of consequences but simply because they know better. Yet in anonymous contact they don't think twice about it.
It also doesn't help that these interactions are ephemeral. Tomorrow you play against someone else, or chat with a different person. What you said yesterday doesn't matter, you may not even be the same persona. So as far as human relatoins go, they know that the chat interaction isn't a human relation, it's a short anonymous interaction.
So I think the important thing for us parents to convey is that being an asshat isn't ok just because you are anonymous. It might help to explain that everthing typed anywhere online is usually recorded forever, and that true anonymity doesn't exist either. That, and of course that real people deserved being treated like real people, even when anonymous.