Intent is not magic. The hurt caused is not just mended because you didn't mean it.
Maybe this is a better way of putting it: Person A and Person B exist. One of the two means to be sexist, and one does not. They both go around and do the exact same things.
What is the difference between A and B, as far as women or our community are concerned? There effectively isn't any. Now, maybe Person not-sexist is more likely to come to understand how their actions are harming people, and be reformed, but before that point, their intent doesn't mean a damn: they're still hurting others.
It's strategic. It's because we're trying to correct behavior, not just punish offenders. We live in a society together and you can't just push these people out and marginalize them and expect benefits. This has been discussed to death, and the consensus is that you address behavior and not identity because attacking identity accomplishes nothing positive.
Which do you think helps remove the behavior in our society: "that sounded racist, is that really what you meant?" or "you're a racist. I can't believe you said that." One of them gives the person an opportunity to evaluate what they said and how they want to be perceived by others, the other immediately puts them on the defensive and marks you and your cause an enemy to their person.
