A survey of men who say they have been forced to have sex by a woman claims blackmail and threats were the most common methods used.
A research project of 154 men "compelled to penetrate" a woman found more than a fifth of them were forced into it by threats and blackmail.
Telling lies, threats to end a relationship, warnings of rumour-spreading and verbal abuse were cited by 22% of men in the survey.
The research project, led by Dr Siobhan Weare, from Lancaster University Law School, and supported by charity Survivors Manchester, also found the use of force, such as pinning down with bodyweight or having a weapon, was reported by 14% of men who completed the survey.
The project aimed to explore one of the "last taboos" - sexual violence by women against men.
Dr Weare said: "Whilst the sample size of 154 may be smaller than typically expected, this must be considered in the context of an issue that is under-reported and under-discussed, and that this is the first and only survey of its kind to be conducted in the UK.
"The 'hidden' nature of this crime and the 'complex' gender dynamics involved means that huge numbers of survey participants were highly unlikely - not because this isn't happening to men, but because many are made to feel too ashamed or feel too distressed to report it."
Dr Weare said it was difficult to know the prevalence of forced-to-penetrate cases in the UK, but research in the US in 2010 found that approximately one in 21 men reported being "made to penetrate" someone else during their lifetime, with 79% of cases involving a female perpetrator.
She added: "The term 'forced to penetrate' has been coined for these cases because, while they involve non-consensual penile penetration, they do not fall under the offence of rape.
"The offence of rape can only be committed by men due to the requirement of penile penetration of the victim. In 'forced to penetrate' cases, the offender is the one being penetrated by a non-consenting victim."
All 154 of the UK male survey participants had experienced compelled penetration.
The men shared their most recent experience of being forced to penetrate a woman, as well as their engagement with the criminal justice system, how they would label what had happened to them, whether they had experienced multiple victimisation, and emotional and psychological harms.
The majority of the participants who completed the survey reported that they knew the woman, often as an acquaintance or a friend, and just over half were in, or had been in, a relationship with the perpetrator.
Only two men said that they had reported their experience to the police and in both instances the case did not make it to court.
"Rape" was the most frequent label used by the participants in describing their ordeal, despite the law not recognising such cases in this way.
"Sex" was used least frequently.
The vast majority, 80% of them, did not disclose their experience to family or friends and 74% had not sought support, suggesting that men are left feeling isolated and alone in dealing with their experiences.
Duncan Craig, the founder and chief executive of Survivors Manchester, a charity supporting males who have experienced sexual violation, said: "This really is a groundbreaking piece of work by Dr Weare.
"We have got to break the silence on this and let men know that we are here to listen and support them when needed."
The research project was funded by a Lancaster University Faculty of Arts and Social Sciences Knowledge Exchange Grant.