Here's what I've learned so far from the furore over the rape allegations against Julian Assange: when it comes to rape, the left still doesn't get it.

The problem is not that many WikiLeaks supporters question the zeal with which Swedish authorities are pursuing Assange. Maybe it's true that an ordinary guy, faced with similar accusations, would have been allowed to slip away quietly once he left Sweden rather than become the subject of an Interpol red notice. (Maybe not, though. The 11 Swedes on Interpol's public red list include people wanted for fraud and other non-spectacular crimes. Much has been made of the fact that only one of these, an alleged child molester, is charged with a sex crime. But the vast majority of wanted people are privately listed, so actually there's no way of knowing if Assange's case is exceptional.)

Given that US politicians, from Joe Biden to Sarah Palin, have called for Assange's head, it isn't paranoid to suspect that he is being singled out in order to extradite him to the United States. But it could also be that Sweden is following up because law enforcement officials get mad when world-class celebrities flee the country and then thumb their noses at them – cf Roman Polanski.

What's disturbing is the way some WikiLeaks admirers have misrepresented the allegations, attacked the women and made light of date rape. It's been known for some time that Assange was accused of using his body weight to force sex on one woman, ignoring her demand that he use a condom, and penetrating the other woman while she slept, also without a condom, despite her wishes; yet writer after writer has treated the whole thing as a big joke. It was "sex by surprise" – some arcane Swedish thing – wrote Dave Lindorff on Truthout. Also, Assange didn't tell the second woman about the first and didn't return her phone call. Hell hath no fury like a groupie scorned.

Appearing on Keith Olbermann's show after he put up $20,000 to help bail Assange out of a British jail, Michael Moore – apparently an expert on Swedish rape law – called the case "a bunch of hooey": "the condom broke during consensual sex". Olbermann made matters worse when he retweeted Bianca Jagger's tweet linking to a blog post by Mark Crispin Miller, claiming that Assange accuser "Ms A" had "interacted" in Cuba with an anti-Castro women's group supported by terrorist and former CIA agent Luis Posada Carriles, and had published anti-Castro "diatribes" in a Swedish magazine. You would think the left would be more sensitive to charges of guilt by association – since when did marching in a demonstration mean you signed on to everything its supporters support? By those lights, everyone who went to an Answer-sponsored march against the Iraq war thinks North Korea is a Marxist paradise.

And everyone who believes and promotes the "information" that "Ms A" is a CIA "honeytrap" is an antisemite and a Holocaust denier – because the original source for that story is one Israel Shamir, writing in Counterpunch and vigorously defended by Counterpunch editor and Nation columnist Alexander Cockburn, who also belittles the accusations as "unsafe sex and failure to phone his date the following day".

I spent a few hours on israelshamir.net and learned that "the Jews" foisted capitalism, advertising and consumerism on harmonious and modest Christian Europe; were behind Stalin's famine in Ukraine; control the banks, the media and many governments; and that "Palestine is not the ultimate goal of the Jews: the world is." There are numerous guest articles by Holocaust deniers, aka "historical revisionists". We have now produced on the left an echo chamber like that on the far right, where the scurrilous charges of marginal fanatics are disseminated through electronic media and end up, cleansed of their original associations, as respectable opinion.

The heroic Sady Doyle, a blogger at Tiger Beatdown, gets lots of credit for starting a Twitter campaign that forced Moore and Olbermann to – sort of – back off their sexist chortling. But it's too late: the "revelations" that Sweden has laws against condomless sex and that "Ms A" is a CIA "honeytrap" are all over the left blogosphere.

And it isn't just men who are spreading it. On the Huffington Post, Naomi Wolf posted a satirical letter to Interpol, aka the "world's dating police", repeating the broken-condom falsehood and adding that Assange's crimes include "texting and tweeting in the taxi… while on a date and, disgustingly enough, reading stories about himself online" in the cab. Is this the same Naomi Wolf who wrote a 2004 New York magazine cover story accusing Harold Bloom of putting his hand on her thigh 20 years before? Wolf argues that the accusations against Assange demean the seriousness of rape.

In fact, Swedish law does distinguish among degrees of rape, with Assange being accused on one count of the least grave kind. In a much-cited letter to the Guardian, Katrin Axelsson of Women Against Rape argued that Sweden's low rape conviction rate proved that Assange was being set up – in 2006, she claimed, only six people were convicted out of 4,000 reported. Not so.

"I don't know where they got those figures," Amnesty International's Katarina Bergehed told me by phone from Sweden. "In 2006 there were 3,074 rapes and 227 convictions." (Sweden tracks rape by individual acts, not by number of victims, so the prevalence of rape is less than it looks.) Bergehed should know: she wrote the Swedish section of the Amnesty report on sex crimes in the Nordic countries that Assange supporters cite as proving that Sweden is the worst place in Europe for rape victims.

One reason the Swedish rape conviction rate is low is that – thanks to 30 years of feminist progress – the law defines sexual violence and coercion broadly but, as in other countries, police and juries often do not. The same seems to be true of large swaths of the American left.

WikiLeaks is revealing information citizens need to know – it's a good thing. Assange may or may not have committed sex crimes according to Swedish law. Why is it so hard to hold those two ideas at once?

This article first appeared in The Nation

• This piece was amended on Monday 27 December at 14:13 to anonymise the names of the two women in question