Oh dear, Slutwalk London. On Saturday you're marshalling crowds of women in fishnets and bras to chant "my dress is not a yes" and promoting petitions insisting that the Home Office should prosecute rapists. Come Tuesday, you're taking to Twitter to issue statements objecting to the extradition of Julian Assange to face rape allegations in Sweden. Rapists should be prosecuted, but according to Slutwalk London, the fact that many who are accused of rape ultimately aren't convicted means that this particular accused rapist shouldn't be subject to due process. It's an awkward position to adopt, and the most awkward thing of all is the way it conscripts those who joined the march to a cause that was never part of the prospectus.

I've squinted at the statement, lurched back and forth at my monitor, and made myself boss-eyed in the hope that the statement is the literary equivalent of a Magic Eye picture: looks like a nonsensical smear on first glimpse, eventually resolves into glorious meaning. I'm going to save you some time here. Don't bother with any of that, because the statement really is nothing but a self-contradictory trainwreck. Slutwalk Toronto (the mother of all Slutwalks) has already discreetly expressed its concern about the London branch's statement, again via Twitter.

Now, the legality of Assange's extradition has been done and done and done again, and I've got no desire to rehash here what's been dealt with thoroughly elsewhere – not least because I'm not a lawyer, and I think the world can do without another non-legal opinion on this business. But given that Slutwalk London has no more passed its bar exams than I have, I do wonder why on earth they felt they had to say anything.

What did the person posting that statement think she was going to achieve? It's hard to imagine that anyone was waiting for Slutwalk London to take a stand before they decided for themselves about a matter of international law. There are things I might look to Slutwalk for guidance on: for example, does flashing my thigh mean I deserve to get raped (no) and should the police advise women against flashing their thighs for their own protection (no again), but the Assange case isn't one of them.

And while it can't do any good, it can certainly do plenty of harm. Anyone who's ever had a vague feeling that all-out-with-our-tits-out is flawed as an anti-sexual violence strategy could point to this as evidence of Slutwalk's lack of coherence – and no one who's ill-disposed to a cause is likely to distinguish London's organisers from all the Slutwalks worldwide making a noise for women's right not to be assaulted.

At the same time, Slutwalk London has inadvertently lined itself (and its unwitting supporters) up with an unappealing gaggle of rape apologists and victim blamers. It's all very well for the statement to stress "We are not saying the women lied or that they should not get justice," but lots of people who support Assange have said that women lie. If an anti-rape campaign must intervene on this case at all, surely it should be addressing those grotesque statements, not condoning the position of those who made them.

The revelations of how the victims of the Rochdale child abuse circle were failed show again how prejudices about female sexuality are used to deny women and girls justice. In a BBC interview, one of the abused girls explains that social services told her parents that she was a "prostitute" and had made a "life choice" – even though she was only 15. Apparently, whatever she wore and wherever she went were seen as tokens of her own moral failings, rather than being recognised as the signs of hideous abuse. Attitudes like that are what Slutwalk was founded to counter. Co-opting supporters in defence of an alleged rapist is a betrayal of victims of sexual violence and a manipulative abuse of the backing that women have given to the Slutwalk cause. Oh dear, Slutwalk London, oh dear.

Slutwalk London were asked for a comment but have not yet replied.

• This article was amended on 28 September 2012. The original version referred to SlutWalk London objecting to the prospect of Julian Assange being extradited to face rape charges in Sweden. This sentence has now been changed to remove the implication that charges have already been brought