Image copyright Getty Images

A man is challenging his conviction for raping a woman who willingly slept with him after he falsely claimed to have had a vasectomy. How can someone be guilty of rape if their partner has agreed to sex, and what implications does the case have?

"I have a confession. I'm still fertile. Sorry xxx"

Sally - not her real name - was distraught as she read the text message from Jason Lawrance, a man she had met through a dating website. "Are you serious?" she texted back. "You utter bastard. Why the hell would you do that to me?"

Before Sally had sex with Lawrance he told her he'd had "the snip" and she consented to having sex without a condom, but would never have done so if she had known Lawrance was fertile. She also had no idea he was a serial rapist.

Then 42 and already a mother, Sally did not want another child. She took the morning after pill but became pregnant, then went through the ordeal of having an abortion.

Image copyright Derbyshire Police Image caption Jason Lawrance is appealing against his convictions for raping Sally

Lawrance, previously of Leicestershire, went on to be convicted of raping Sally twice - because they'd had sex two times - in a case with no known precedent in the UK.

"Section 74 of the Sexual Offences Act 2003 says a person consents if he or she agrees by choice and has the freedom and capacity to make that choice," said Sue Matthews, the senior crown prosecutor who put the case together. "By lying about the vasectomy he deprived that particular victim of making an informed choice."

"If these convictions are upheld on appeal, the concern is that members of the public, both male and female, who have never been considered criminals in the eyes of the law will be at risk of prosecution for serious sexual offences," said Lawrance's solicitor Shaun Draycott.

Lawrance is far from alone in deceiving a sexual partner to get sex. So could others who do this now face prosecution?

Could removing a condom count as rape?

Image copyright Getty Images Image caption Men's main stated reason for avoiding condoms was to make sex more pleasurable

Kelly Davis, an associate professor at Arizona State University, has surveyed men and women aged 21-30 about "condom use resistance", meaning the tactics employed to avoid using condoms when the other partner wants to. Out of the 313 men participating, 23.4% admitted to having used "deception" at least once since the age of 14.

The most common tactic was lying about planning to withdraw before ejaculation but not actually intending to (19.9%), followed by lying about having been tested for STIs and being STI-free (9.6%).

Dr Davis and her fellow researchers also did focus groups with young men. "The place that these men drew the line was anything involving physical force," she said. "Anything up to that line is just part of the game, because that was how some of the young men in this group viewed it."

Of the 530 women surveyed, 6.6% admitted to having used deception at least once since the age of 14, with the most common tactic being pretending to be on birth control.

Dr Davis and her colleagues also researched the practice of "stealthing", which means agreeing to use a condom then surreptitiously removing it before or during sex. Again, the men surveyed were aged 21-30. Almost 10% of the 626 participating admitted having engaged in stealthing at least once since the age of 14.

"I was shocked that it was almost 10% that reported doing this, and at an average of over three times," Dr Davis said. "So they are not just doing it once - well some are - but many of them are doing it multiple times. It's disturbing."

Twelve per cent of the women surveyed reported knowing that stealthing had happened to them, although some might never have realised it had happened. Many people would not consider stealthing to be rape, but Sandra Paul, a solicitor who specialises in cases of sexual misconduct, believes it is under UK law.

The issue was considered by judges as part of Wikileaks founder Julian Assange's extradition case, and their judgment said sex without a condom would be a sexual offence in the UK if the other partner had only agreed on the condition a condom was used.

What if a man doesn't withdraw?

Katie Russell, national spokesperson for Rape Crisis, believes that lying about the intention to withdraw before ejaculation falls under the legal definition of rape. "That's an example of someone having agreed to sex with conditions, and the other party having reneged on those agreed conditions," she said.

Ms Paul said a such a case has already been considered by a UK court, and it involved a husband and wife. "She didn't want to have any further children and agreed to have sex provided he withdrew before he ejaculated," said Ms Paul. "He seemingly agreed to that but there was sufficient evidence to show he had no intention to withdraw."

The CPS decided not to prosecute the husband for rape, so the wife applied for a judicial review of the decision. "The Admin Court made it quite clear that in their view there was good reason for the CPS to consider charging; what happened was capable of amounting to an offence in that her consent had been negated by his never intending to withdraw," said Ms Paul. "It was sent back to the CPS for them to review their decision based on what the court had said."

The BBC asked the CPS whether the husband was ever prosecuted but it could not provide the information.

Could lying about an STI test count as rape?

Image copyright Getty Images Image caption In one survey of young men, 9.6% admitted lying about having been tested for STIs and being STI-free

Ms Paul thinks lying about having had an STI test is more of a grey area.

"It's difficult," she said. "I think that, probably, if we were taking baby incremental steps from the decision that's been made about Lawrance, as we must do in the absence of legislation, then potentially."

There have already been several prosecutions of men who infected partners with HIV, with Daryll Rowe, Antonio Reyes-Minana and Aaron Sutcliffe being three recent examples. However, they were charged with causing grievous bodily harm, rather than sexual offences.

What if a woman lies about being on the pill?

Lawrance's defence barrister David Emanuel QC compared his client's lie about the vasectomy to a woman lying about being on the contraceptive pill. He argued that if Lawrance were convicted of rape for lying about being infertile, then a woman could arguably be convicted of a sexual offence for a similar act of deceit.

Under current law in England and Wales, a woman cannot be prosecuted for raping a man because rape is defined under the Sexual Offences Act 2003 as an offence of penetration committed with a penis. Legislation in Scotland and Northern Ireland also specifies that rape is committed with a penis.

Ms Russell has a problem with the vasectomy and contraceptive pill comparison. "With issues around contraception and pregnancy, it's the woman whose body and life and health is affected by that kind of lie," she said. "That is not in any way comparing like for like, because it's a woman who has to deal with the consequences of pregnancy and termination, and, in the example given, the impacts on the man are not of a comparable kind."

But Ms Paul is not so sure. "The issue is the extent to which the lie vitiates (negates) consent," she said. "If a man finds himself a father of a child under these circumstances, there are all sorts of consequences that flow from that.

"It might be argued there is a double standard. In so many walks of life we [women] have achieved equality and demand to be treated with the same level of respect as men. Where the lie deemed to create criminal liability is exactly the same, I think it is problematic to carve out areas where we are protected solely because we are women."

What if someone lies about their gender?

Image copyright PA Media Image caption Gayle Newland pretended to be a man, persuaded her sexual partner to wear a blindfold and penetrated her using a prosthetic penis

There have been several prosecutions where women have impersonated men in order to have sexual encounters with other women, or where trans men have not disclosed their assigned gender to female partners. Notable cases include Gayle Newland, Justine McNally and Kyran Lee.

Many such cases involve the use of a dildo and, since rape is an offence of penetration committed with a penis, the charge brought is one of sexual assault or assault by penetration. In the case of McNally, who presented as a teenage boy throughout a relationship with a teenage girl, the Court of Appeal determined that "deception as to gender can vitiate consent".

However, cases like these, sometimes called "gender fraud", have raised questions about the rights of people with gender dysphoria, and whether or not transgender people should have to disclose their assigned sex to partners. Barrister and law professor Alex Sharpe, who has been involved in transgender law reform and activism for over 20 years, is among those with concerns.

"Most of the people prosecuted on the basis of sexual fraud appear to be gender non-conforming, so there's an issue of the singling out of particular kinds of people or transgression for prosecution," she said. Prof Sharpe believes a trans man is not deceiving a female partner if he presents himself as a man, because he is a man.

What about other lies?

People tell all sorts of lies in order to have a sexual relationship, such as lying about their age, pretending to be single when they are married, or claiming to be more wealthy than they really are. However, cases like these have not ended up in court, so jurors have not been asked to consider whether or not such lies negate consent.

Several women have had sexual relationships with undercover police officers embedded in activist groups. One of these women argued this amounted to "a team of officers conspiring to rape", because the officers knew there was no informed consent.

Police have given some of the women compensation, however the CPS declined to bring charges against any of the officers, saying that "any deceptions in the circumstances of this case were not such as to vitiate consent".

Ms Paul believes more guidance is needed about the extent to which lies negate consent - and this will hopefully come in the form of a judgment from the Court of Appeal now the Lawrance convictions are being challenged.

"Ultimately it may well be that legislation is required," she said. "Failing to wear a condom when you said you would, I think clearly on the face of the legislation as it stands, is problematic but an adult lying about their age in most circumstances probably isn't.

"In the absence of specific legislation, who makes the rules about what's in between, and how are juries to navigate these decisions?"

Have you been "stealthed" or has a partner used deceptive tactics in order to have sex with you? Have you used deceptive tactics yourself? Tell us about your experiences by emailing caroline.lowbridge@bbc.co.uk