The ruling body of the Australian Football League is planning to release an educational DVD designed to help players with their attitudes to women. The interactive film was made after a series of sex scandals hit the sport. Some of the scenarios in the film involve alcohol and sex. One section asks the footballer viewer to imagine that a friend’s girlfriend summons him into her bedroom, believing that he is her boyfriend. What should he do? Two choices are offered: a) hop into bed with the girl and pretend to be her boyfriend; b) walk away.

English law has formulated some rules in this area. In a case in 1994, the Court of Appeal held that a rape conviction should stand where a woman had sex with a man whom she mistakenly thought was her boyfriend. Ziani Elbekkay, the culprit, was sentenced to five years imprisonment. The victim, a 30-year old woman, had lived in Hackney in London with her boyfriend for 18 months. Elbekkay was staying the night with them. All three had been drinking. The boyfriend fell asleep in another room. During the night, the woman said she was awakened by someone moving on the bed and touching her. She assumed it was her boyfriend and, without opening her eyes, said “I love you”. Intercourse then began and after about 20 seconds they kissed. Thinking something was not quite right, she opened her eyes and saw that it wasn’t her boyfriend. She punched Elbekkay and cut him with a knife.

In that case, the woman wasn’t too drunk to know what was happening. If, though, a complainant was too drunk to remember whether she (or he, as male rape is the same crime) consented then the prosecution will fall. In 2005, a rape case at Swansea crown court collapsed after a 21-year-old student told the court that she’d been too drunk to remember whether or not she had agreed to have sex with the defendant. She had alleged she was raped by a fellow student in a corridor of a hall of residence at Aberystwyth University.

The defendant, who said that the corridor sex was consensual, had been escorting her to her room from a party after she became very drunk. In court she said if she’d wanted sex she would have opened her flat door and taken the man into her bedroom. But she admitted passing out after drinking too much. Mr Justice Roderick Evans directed the jury to reach a not guilty verdict, on the basis that drunken consent is still consent.

What about where a man makes a drunken mistake that the person he has sex with is consenting? The legal answer, quite reasonably, is that such an excuse usually won’t wash. The underlying reasoning is this: a man can be convicted of rape not only when he has sex knowing the other person isn’t consenting but also where he has been criminally reckless about whether there is consent or not. Therefore, if a man gets so drunk he can’t tell whether someone is consenting he won’t have an excuse because binging itself is reckless.

Technically, though, a defendant can only be convicted of rape if he didn’t “reasonably believe” that the other person consented. That’s to stop sly people who have forced themselves on someone being able to get away with it by saying “personally, I did happen to believe she was consenting” even where such a belief was madly unreasonable. But as being drunk doesn’t necessarily stop someone having a “reasonable belief” (although it usually does), it is just possible for a jury to acquit a man whose slightly drunken belief in consent was nevertheless, in the view of the jury, reasonable. Unlikely, but possible.

One unusual case about mistaken sex arose from an incident in 1971. An 18-year old, had spent the evening drinking with her boyfriend and returned home. She went naked to her bed which was beside an open window. She said she awoke at about 4am to see in the window a naked blond man whose silhouette showed he was in a state of physical arousal. Assuming he was her boyfriend, she beckoned him into her room and they started to have sex. Then she realised “something was different” and she switched on the light.

She was horrified to find it wasn’t her boyfriend. The man testified that he fancied her so that night he had stripped naked (except for his socks), found a ladder and gone up to her window where, he said, he’d been surprised she invited him in so readily. He was charged with a type of burglary that requires a defendant to have “entered premises” as a trespasser with the intent of committing rape.

So, the key point was whether the man, or any part of him, had entered the bedroom from his perch on the window sill before the young woman had invited him in. If he had anatomically entered the premises uninvited, even by an inch, he’d be a trespasser and thus guilty of burglary. His conviction was eventually quashed. "The point", as the court delicately put it, "is a narrow one, as narrow as the window sill".

Professor Gary Slapper is Director of the Centre for Law at The Open University

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