The unmasking of Harvey Weinstein was like the breaching of a dam, against which decades of abuse, outrage, injustice, rumour, shameful complicity of employees, and unwarranted shame from victims had built. The awesome power of its bursting has already completely changed the territory, obliterating not just Weinstein’s nefarious power but all the assumptions and understandings by which a woman’s discretion was ensured.

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The expectation previous to this was that victims of harassment and abuse would accuse themselves, efface their own feelings, and stay if not completely silent, then silent where it counted – in the public sphere. Allied to this was the assumption that bystanders had no part to play, since if the target didn’t object, who is to say that there was anything to object to? There were countless codes to reduce the seriousness of this behaviour, from “it was a long time ago” to “he does that to everyone” – a neat little manoeuvre by which the serial sexual predator was exonerated by his sheer promiscuity: if he did it to everyone, how could he have meant anything by it?

All that has been upturned. If there’s one thing more astonishing than how long a corrupted system can stay upright, it’s how fast it disintegrates once the rot sets in.

Post-Weinstein, the “me too” movement must surely have scalps yet to collect. At the moment many women stand on the brink, ready to share their experience, not yet ready to name the perpetrator. But implicitly, if you accept that solidarity starts with openness – if you accept that, as a victim, you have nothing to be ashamed of; if you accept that sexual exploitation is unwanted and unnecessary; if you accept that nothing protects a perpetrator quite like silence – then sooner or later you owe it to your own testimony not just to share, but to accuse. The French version of #metoo makes this explicit: #balancetonporc – grass up your pig. Their feminism has always been more philosophically absolute, more ready to follow itself to its logical conclusions, than the Anglo-Saxon variant.

So far there have been only a handful of other high-profile cases in Britain: Max Stafford-Clark, whose departure from his own theatre company, Out of Joint, predates the Weinstein affair, but has enough in common to join it as a trend; and the journalist Rupert Myers, let go from GQ after “online allegations”. It is hard to imagine it ending here; there must be many men who know it’s a case of when, not if.

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There’s nothing quite like the uncomplicated thrill of seeing justice finally visit those who thought they were above it. But this new mood brings with it more difficult ethical scenarios than simply unmasking the villain, for feminists in particular. We had five minutes of being Scooby Doo, before the left got the spotlight. The website LabourToo launched for people to anonymously share their experiences of abuse in the party, John Mann pledged to speak up as a bystander, and the Clive Lewis row erupted.

Those who don’t watch live-streamed debates by the leftwing news outlet Novara Media, who don’t follow the Tory minister Justine Greening on Twitter, and who are quite accustomed to hearing Jeremy Corbyn condemn something without knowing exactly what he’s on about – there must be a couple of you – may be unfamiliar with the finer details. Lewis, the MP for Norwich South, was appearing with my colleague Dawn Foster on a Momentum panel on the fringes of the Labour party’s annual conference. It was compered by Ash Sarkar, a Novara editor. There was a gameshow segment, Sarkar asked an audience member to score, and to do it so that everyone could see. The scorer made a face, and Lewis said: “On your knees, bitch” – a video of which emerged last week and was broadcast by the rightwing blog Guido Fawkes.

Senior Labour MPs – Harriet Harman, Jess Phillips – were dismayed; Corbyn said the comments were “completely unacceptable, completely wrong, not acceptable under any circumstances at any time”. The Conservative MP Nusrat Ghani has now vowed to ask the speaker for an “urgent debate” tomorrow, on the basis that Lewis “used position of power & establishment to undermine parliament”.

When objecting to misogyny is a niche business – just old lefties from the 1980s and young fourth-wavers, like a political crochet circle – there are many downsides, the main one being that no one listens to you and nothing changes. But it has one upside: as feminists, you get to set the parameters. You get to decide what’s a joke and what isn’t; who is an ally and who is an enemy masquerading as one; what macho language sets out to degrade women, or is just trying to impress after too many beers. Your arguments have very little power outside your own political circle, but you have a lot of power within it, the power to act on your own discretion and exempt some people – broadly, the ones you like – from your own rules.

Now that the message has broken through, we’re having to be schooled in gender equality by Guido Fawkes

Frankly, the toleration of sexist language and behaviour on the left is a long-term problem, for precisely this reason: as the inventors of “safe spaces” and sisterhoods, we were our own judge and jury. It created some bizarre double-standards – female Labour MPs facing hideous misogynistic abuse for being insufficiently supportive of a leader who probably hates misogyny as much as he hates unbridled free markets, to take a wild for-instance. But it also left a lot of space for playfulness, for jokes, for missteps, for human error. It is a breakthrough that it is suddenly common sense that misogyny is never casual, but celebrating that involves accepting that the left are no longer the arbitrators. It was right for Corbyn to make a statement so strong about Lewis. If Jacob Rees-Mogg had said it, we would have dug out our pink hats and had ourselves a parade.

Yet it’s a bitter pill, to be schooled in gender equality by Guido Fawkes, and lectured about the misuse of the “power of the establishment” by a member of the party seeking to accrue and misuse more establishment power than any government in living memory. Their only interest is the “gotcha” moment. They seek to turn sexism into an undifferentiated offence, one huge morass, where a joke is the same as harassment, which is the same as assault, and the wall of silence is rebuilt on different foundations: a problem is so large, it cannot be scaled.

The debate for the left, and feminists on the left, remains the same: how do you build a movement on the principles of equality without letting the self-satisfaction of that ambition blind you to the faults of your own culture? Whatever the answer is, it won’t be, “Huh! Men – they’re all the same.”

• Zoe Williams is a Guardian columnist