In the first 48 hours after the guilty verdict in the Grace Millane murder case, Fiona Mackenzie received 50 interview requests from the media in the UK, US and New Zealand. In the previous week, as Millane’s killer claimed she had died in a “sex game gone wrong”, Mackenzie, founder of We Can’t Consent to This, which campaigns against the “rough sex” defence, had managed to recruit a voluntary press officer to help with the deluge. Between them, they had juggled BBC Breakfast, ITN, Sky, Channel 5, local TV and radio stations, with Mackenzie also covering her full-time corporate job as an actuary.

“My employers have been very accommodating,” she says. “I’m still claiming that this doesn’t take up much time – though I’ve had to give up computer games and reading novels.” She’s not complaining about the extra work. “For the first half of this year, it was a real struggle to get much interest at all. I think the Grace Millane murder has really changed things.”

It wasn’t just the murder that has sparked rage – though the details are horrifying. Millane was a British graduate who arrived in New Zealand on a round-the-world trip; she agreed to a date with someone she matched with on the dating app Tinder, then was strangled. Later she was contorted into a suitcase and buried in the woods – though not before her killer had photographed her naked body, watched some porn and gone on another date. Despite these horrifying facts, the trial focused on Millane herself, her sexual history and use of dating apps like Tinder and Whiplr, shifting responsibility away from the murderer and on to the victim.

Newspapers ran headlines such as “Naïve and trusting”, “Strangled tourist liked being choked” and “Grace Millane ‘encouraged date to choke her during sex and apply more force’”. “After the murder, the trial and the way it was reported,” says Mackenzie, “I think there’s a new understanding of the ‘rough sex’ defence and a level of anger that wasn’t there before.”

What Mackenzie calls the “rough sex” defence is the claim in murder cases that the victim consented to violent sex, which led to their death.

Mackenzie launched We Can’t Consent to This last December, building the website over Christmas. “To be honest,” she says, “I’d gone on holiday on my own and was quite bored.” She was soon joined by a handful of volunteers – an old school friend, some fellow Lib Dem campaigners and some women she had met on Mumsnet: “I don’t have children but had originally gone on looking for running tips and found an incredible group of feminists.”

Natalie Connolly ... her brutal killing in 2016 prompted Mackenzie to launch We Can’t Consent to This. Photograph: Staffordshire Police/PA Wire

The trigger for Mackenzie was the case of Natalie Connolly who was brutally killed in December 2016 by John Broadhurst. Broadhurst, her partner of a few months, left the 26-year-old bleeding to death at the bottom of the stairs in the home they shared with her eight-year-old daughter. Connolly suffered multiple blunt-force injuries but Broadhurst claimed it was the result of “rough sex”, and was found guilty of manslaughter. He was sentenced to three years, eight months. Earlier this month, he appealed to have his jail time cut but was unsuccessful. “Some people raged against the verdict but in the main, there seemed to be astonishing levels of acceptance,” says Mackenzie. “People somehow believed it, saw it as a weird one-off and thought the victim must be to blame for the riskiness of her behaviour.”

For Mackenzie, this stirred difficult memories of a case that took place near Aberdeen, where she was at university studying maths, in 2000. The body of Mandy Barclay, a 32-year-old mother of two, had been found in the local woods. She had died of asphyxia and severe rectal injuries, and her husband, Niall McDonald, was charged with murder. In court, the defence claimed that Barclay had died while practising the kind of sex “that narrow-minded people would call kinky”. As a student, Mackenzie followed the details offered up by McDonald (whip, French maid outfit, rooftop sex) and when he was found guilty of the lesser charge of culpable homicide and sentenced to seven years, Mackenzie admits that she believed on some level the victim was somehow culpable.

“With the Broadhurst trial, I felt people were reacting the way I had,” she says. “I wondered how many other cases there were and wanted to bring them together to show they weren’t isolated incidents.”

This wasn’t easy. There are no official statistics. Cases aren’t collected or categorised under the defence, so Mackenzie has trawled through media reports and legal archives to find them. By the end of that first Christmas holiday, she had 35. She is now aware of 59, although the true figure is probably higher – and doesn’t include the vast number of non-fatal alleged assaults in which “rough sex” was argued as a defence. The homicides listed on We Can’t Consent to This stretch back to 1972, each one a terrible testament to lessons not learned. “Though pleading consent has no status in English law, juries seem to accept it,” says Mackenzie. “We’ve found that it has resulted in a lesser charge, lighter sentence, or acquittals in 45% of the cases.”

Grace Millane ... ‘Her murder has really changed things.’ Photograph: Auckland City Police /PA Wire

In 1979, to pick one example, 19-year-old Vivien Scott was strangled by 21-year-old DJ John Dudgeon (aka John Taylor) in what he claimed was “slap and tickle”. Dudgeon’s defence argued that his “one mistake” had been not seeking help immediately. He was found guilty of manslaughter and sentenced to four years. After serving just 17 months he was out, and attempted to rape and murder a woman in her home. When released a second time, he murdered 32-year-old Susan McNamara.

In another case from 1991, Stuart Williamson received a three-year sentence for the manslaughter of his girlfriend Honor Matthews, 20. Despite his previous convictions for violence, his defence claimed that the “deeply attached” couple had been engaged in “pseudo-masochistic’ ‘neck compression” aimed at “giving pleasure”. After his release, Williamson abused his new partner and killed his mother.

One of the earliest recorded “rough sex” defences took place in 1961 and even this carries overtones of the Grace Millane murder. It took place in Kenya and involved a man called Sharmpal Shikh who claimed his pregnant wife Ajeet had been killed during a “sexual embrace” – she had died from internal injuries to the neck and chest. He had then embarked on an elaborate deception, taking Ajeet’s body to the courtyard and stabbing her to make it resemble a robbery. As in the Millane case, the prosecution argued that the behaviour after the killing was the action of a murderer, while the defence countered that it was done in fear, shock and panic. Shikh was believed and the verdict was manslaughter. Rather like Broadhurst, he made an unsuccesful appeal to be acquitted of this.

Seeing parallels and repeats through the decades, it’s clear that the “rough sex” defence often sits within a long history of blaming women for their own killings, of looking away from perpetrators to see how victims brought it on themselves. Claiming they actually consented is the logical endpoint.

Hallie Rubenhold, social historian and author of The Five, the untold lives of the women killed by Jack the Ripper, has no doubt there’s a pattern here that has been going on for centuries. In her book, which last week won the Baillie Gifford prize, Rubenhold re-examines the lives of the ripper’s victims. She finds no credible evidence that three had ever worked as prostitutes, but shows instead how all, born female and working class, lived and died with the cards stacked against them.

The press coverage at the time was often wildly inaccurate, and the entire mythology around “ripperology” since has painted the victims as vulnerable prostitutes, faceless “fallen women” who lived recklessly. After one of the murders, a letter to the Times from a senior civil servant even thanked the “unknown surgical genius” for clearing the East End of its “vicious inhabitants”.

“What it came down to, just as it did with Grace Millane, is victim-blaming,” says Rubenhold. “It’s so deeply ingrained, it’s almost in society’s DNA. Instead of looking at what happened and why, we’re geared to look at the women instead. Just as rape victims are asked about their sexual history, what they were wearing, what they had been drinking, the knee-jerk reaction with Grace Millane was, ‘Oh well, she was far away, on Tinder, looking for a hook-up – what kind of woman does that?’ The whole point is it doesn’t matter. They don’t deserve to die.”

Why, finally, are we waking up? Mackenzie believes it is partly the rising tide of cases. There has been a 90 per cent increase in the last decade.

More than this, though, is the growing sense among women that this could happen to them – we might all be one “bad date”, one “Tinder match” away from dying like this. While the Millane trial was dominated by male voices – the judge, defence, prosecution, former boyfriend – the fury on social media came from women. A typical tweet with the #gracemillane hashtag read: “If I’m ever murdered by someone I’m having sex with, I’d like it on record that I will not have consented to being choked so hard that I DIE.”

In the last fortnight, Mackenzie has added an “our stories” section to her site with women sharing their own experiences of assault and “rough sex”. Over 100 have contributed. One describes falling in love with an “educated, gentle, kind professional gent” who could only climax with his hands round her neck. Another only managed to free herself from a choke-hold by hitting the man with a lamp: “He was really upset – had thought we were having ‘great sex’,” she writes.

Change is coming, however. The MP Harriet Harman is confident that her two amendments to the domestic abuse bill – designed to reinforce the fact that consent can be no defence for death – will be seen through in the new year, despite the unlawful suspension of parliament in September and the election causing frustrating delays.

Meanwhile, Mackenzie is chasing the Crown Prosecution Service to properly log and track these cases and enforce Harman’s amendments when they are passed. “Changing the law is the easy bit,” she says. “The hard part is making sure it works in practice. I also want to keep raising awareness and pushing for policy responses to the appalling normalisation of unbidden violence against women during sex.” Next year, she thinks she will formalise her campaign with official charitable status, because relying on a handful of volunteers is becoming tricky. “I’m not a professional campaigner,” she says. “I’m learning as I go.” How does it feel to know people are now listening? “I’m just really relieved,” she replies.

• This article was amended on 28 November 2019 to remove an incorrect assertion that the founder of We Can’t Consent to This uncovered “two so-called ‘rough sex’ killings from 1996. A decade on, she has found 20 a year.'” Those figures related to cases involving deaths and injuries to women, not just deaths, in 1996 and 2016.