A man who strangled a vulnerable woman during sex after drinking 24 bottles of beer has been sentenced to four years and eight months in prison.

Sam Pybus, 32, from Darlington, pleaded guilty to manslaughter after accepting he had killed Sophie Moss by applying pressure to her neck during consensual “rough” sex, but that he did not intend to hurt her.

Campaigners said the sentence sends a “dreadful message to women”.

“It seems that strangling a woman to death is still viewed in law as an unfortunate accident, rather than terrible serious violence,” said a spokeswoman from We Can’t Consent To This, which campaigns against the use of “rough sex” defences in cases involving violence against women and girls.

Earlier this year the government clarified the law to restate the broad legal principle “that a person cannot consent to actual bodily harm or to other more serious injury or, by extension, to their own death”.

Moss, a 33-year-old mother of two, was described in court as “a vulnerable individual who endured poor physical and mental health”.

At the time of her death in February this year, her children were living with their father and she lived alone. She was in a relationship with another man, but had been having consensual sex with Pybus on and off for three years.

Pybus, who was married, turned himself in at Darlington police station in the early hours of 7 February this year.

Richard Wright, QC, prosecuting, said Pybus told detectives that “he must have strangled [Moss] but couldn’t remember doing so”. He said that he woke to find her naked and unresponsive, but instead of doing any first aid went to his car and thought about what to do for 15 minutes before driving to the police station.

A postmortem found she died from strangulation. The pathologist said her injuries “do not suggest either very prolonged or very forceful strangulation or strangulation which was actively resisted”.

Wright told Teesside crown court that Pybus claimed sex between him and Moss was “always rough and that he would dominate her during their sexual activity but that he would never go so far as to hurt her”. He said he would sometimes apply “mild pressure” to her neck during intercourse: “an act that she encouraged and enjoyed.”.

Pybus said he arranged to go to Moss’s house to have sex on 6 February this year, after his wife had gone to bed. He told police he had been drinking for 10 hours and had consumed around 24 bottles of Amstel lager before getting in his car to visit Moss.

He purported to have little recollection of the circumstances of the sex leading up to her death. He said he believed he must have strangled her because his hands were hurting, but couldn’t remember what happened.

Wright told the court the prosecution had “not accepted any form of ‘defence’ advanced by the defendant”. Rather, it was a case in which an allegation of murder could not be proved beyond reasonable doubt.

Earlier this year justice minister Alex Chalk said: “Perpetrators of these crimes should be under no illusions – their actions will never be justifiable in any way, and they will be pursued rigorously through the courts to seek justice for victims and their families.”

But We Can’t Consent To This said the case showed the law was not working. “This sends a dreadful message to women – four years, eight months is an outrageous sentence for killing a woman,” said a spokeswoman. “We don’t think this is how the law should work – and look to the government now to see what they intend to do.”

Moss’s brother James, told Teesside crown court that his sister was “joyous, vibrant, funny, talented and fearless, unless she saw a spider”. The family would “never be able to shake the belief” that she was “a victim, taken advantage of and exploited, and was subjected to an entirely avoidable and infinitely tragic end.”.