Despite the heartening news that a Kingston jury returned not guilty verdicts in a landmark porn trial on Wednesday, grave questions arise about the right of the state to intrude on the privacy of the individual with unfounded obscenity prosecutions.

My client, Simon Walsh, was charged with five counts of possessing "extreme pornography" under section 63 of the Criminal Justice and Immigration Act 2008 (CJIA). The images showed acts of fisting and urethral sounding (the insertion of surgical rods into the urethra for the purpose of sexual gratification).

These acts are legal to perform (fisting is even discussed in Fifty Shades of Grey - now the bestselling book in British history), but representing them is criminalised under the CJIA if they are "likely to result, in serious injury to a person's anus, breasts or genitals". While prosecutions under s63 are not uncommon (1,337 offences were prosecuted last year), most accused plead guilty. It is therefore rare for a jury to consider this specific situation: whether someone is guilty of a crime that could result in imprisonment due to possessing images of legal acts.

Simon is openly gay and enjoyed these acts in private with other consenting adults, so what has any of this got to do with the general public? Like Simon, you could be sent a potentially illegal picture via email. One which you never requested or opened, only to later find yourself in court, accused of being in possession of it.

While some defendants have hundreds of thousands of allegedly extreme or indecent images on their computers, Simon had five images of consensual adult sexual activity and a single unrequested picture unopened on his email server attached to an email containing a story about "Jason". Simon never requested this image and the prosecution were unable to prove Simon ever opened, viewed or knew it. In the story Jason was described as being in his mid twenties. The prosecution described Jason as being 14 years old; the legal age of representation in pornography being 18.

Three defence experts viewed the image and stated in written reports that Jason was in his twenties. As a matter of legal procedure, the jury never heard this expert evidence. Instead they used their common sense and acquitted Simon.

Nonetheless this procedure raises significant concerns about the way in which prosecutions are conducted and the right of the state to interfere with the privacy of the individual.

Unlike the Obscene Publication Act, which covers distribution, the CJIA shifts the burden on to individuals in possession of pornography. People need to know how to modify their behaviour in accordance with the law, yet it is unclear what acts constitute "extreme pornography".

As well as criminalising acts which are legal to perform, the CJIA would seemingly outlaw images which have been exhibited in art galleries. In 2008, the Barbican Gallery's Seduced exhibition included photographic images of male on male anal fisting and male urethral insertion from artist Robert Mapplethorpe's X Portfolio. The gallery cleared them with the City of London police before the exhibition opened.

Police also regularly misclassify images they discover. In the Stafford extreme pornography trial, my client was initially charged with being in possession of over 1,250 "extreme" images. Upon viewing them, it became clear that over 900 were of clothed performers not engaging in any form of sexual activity. That defendant was eventually acquitted by a jury of all charges.

So, would you even know what material might be illegal? Have you been sent junk emails and not opened them? How much do you know about operating systems caching images attached to unopened emails?

Simon is a barrister specialising in actions against the police, an alderman of the City of London, a magistrate and was formerly an aide to the Mayor of London. As a consequence of this prosecution, Simon was fired by Boris Johnson and excluded from his chambers. He ran the risk of going to prison. He had to listen to his innermost sexual fantasies discussed in court and media. This included an inaccurate accusation in the Mail Online that he had actually viewed "child porn".

In questioning defence expert witness Dr Clarissa Smith, prosecution counsel Thomas Wilkins asked if those who attended sexual health clinics were significantly more likely to have engaged in "risky" sexual behaviours. She disagreed.

He then stated that if the fisting pictures were of women they must be degrading and objectifying. She disagreed. In his closing address to the jury, he described Dr Smith's evidence as: "disingenuous, self-serving and dishonest".

In addressing the jury, defence counsel Matthew Buckland stated that: "The modern zeitgeist is an inclusive one. To call this obscene is a huge retrograde step". Nevertheless in his summing up for the jury, His Honour Judge Price QC stated that: "It is emphatically not your duty to send out a message to anyone about modern society".

Thankfully, the result of the jury's verdict is precisely that. They have made a powerful statement about the voyeur state intruding on the liberty of individuals' private and consensual sex lives. As the American comedian Bill Hicks said in satirising the US supreme court's attitude:

"The problem with pornography is basically this: no one knows what it is. Other than that we got a really good grasp on the situation. We know it's bad; we just can't figure out what the fuck it is."