I've seen too many cases where subjective approaches are used to judge such things, and what one person perceives as racist or sexist is not so perceived by another person. It really does vary from person to person, and for the big banhammer to be used requires objective rather than subjective interpretation. The idea of getting rid of sexism and racism is good, the implementation varies between too lax and too restrictive due to subjective interpretation. Who decides if something is offensive? Can you get a person banned because you take offense at a comment, or only if the administrators agree that the comment is offensive?
Then there is the single comment vs pattern of behavior argument. There is already the block/mute function. If a sufficiently large number of people block or mute a person, that should flag them for the administrators to check. If the administrators view it the same way, then warn them that their comments are in violation of the rules. If they continue, ban them, by all means. But to have them banned because of a single comment rather than a pattern of behavior is too harsh, in my book.