(Although I'll concede that usage of the term 'slut' is problematic at best).
The answer is this: because a better approach to sex means changing the way we currently view sex, but doesn't involve criticizing or minimizing sex or sexual expression. Sluts are not the problem. Eliminating harassment by eliminating sluts reinforces the problem, it does not undermine it.
Conservatism is easy. It's convenient. It's a lot easier to tell women to expect inappropriate attention if they dress provocatively than it is to tell men that harassing women is wrong regardless of the circumstances. But only because that's the way it's been for so long.
But that's not right. Women deserve to be able to choose how to express themselves, but they do not deserve to be harassed for it. That's why I'll always support a woman's choice to, for example, dress like a slut. That doesn't mean I condone the harassment that's heaped on her for it. God no - the contrary is true.
I despise the harassment, but for me the solution isn't to stop women from 'inviting' it, but to stop men (and other women) from believing it's okay to dish it out. And the way to do that is NOT to tell women to stop dressing provocatively, but to focus on why the harassment is wrong (while simultaneously celebrating women who have the courage to express themselves).
Yes, women sometimes dress like sluts today and they often get harassed for it. This is a symptom of our sex negative culture. I want women to keep dressing like sluts, but I don't want them to be targeted by so much harassment. Telling them to stop dressing like sluts is indirectly condoning the harassment they receive in the sense of saying they deserve it for dressing that way. They don't deserve it! Being sexy is not license to be treated like shit! Are we puritans here?
Preventing harassment by telling women not to dress like sluts does NOTHING to combat the poisonous attitude many have which enables them to shamelessly commit that harassment in the first place. I want to live in a world where women can express themselves confidently and sexually, and be appreciated for it, without fear of harassment and inappropriate attention (which is defined not as sexual attention, but as bothersome attention, whether sexually oriented or not).
That's why I support SlutWalk, and why I support women who dress like sluts. Because they deserve to be able to choose to do that and be celebrated for it, not condemned. If you condemn SlutWalk and what it stands for, you are participating in a sex-negative culture of slut-shaming. That's not feminist, and it's not right either.