It is said that whether sexism is intentional or not does not matter - it is

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It is said that whether sexism is intentional or not does not matter - it is sexism all the same. How does that work, though? We generally distinguish between, for instance, killing someone by accident (as in manslaughter) and killing someone on purpose (murder, especially premeditated murder), and we generally agree that the longer or more intensely a crime has been intended, the worse it is. Why, then, should accidental sexism be just as bad as sexism that is intentionally perpetrated by somebody who explicitly approves of sexist ideas?