Although I applaud the audacity and bravery of the women who decided to identify as men and use the mens' changing room at the Dulwich Leisure Centre, sadly their campaign is misguided and is likely to lead to harm to cisgendered women.
The only law in England and Wales that gives someone who was born male the right to use female spaces, such as changing rooms, is the Equality Act. On the face of it the act only applies to those who propose to undergo, are undergoing, or have undergone a process to reassigned their gender. To understand how it is interpreted though you need to look at the case law. The leading case is Croft v Royal Mail and although it applied to an earlier statute in a slightly different context, it is still good law. This case held that a transwomen would only acquire the right to use the women's toilet at work when she was significantly advanced in her transition.
So the law today does not allow men (or women) to self-id their way into the opposite gender's changing room. And the government has stated that it is not going to change the Equality Act so the law in this area will not change. And as far as I know, no pervert has succeeded in using the Equality Act to access a changing room in the 7 years that the Equality Act 2010 has been law.
The government has announced however that it is considering simplifying the process of changing legal gender under the Gender Recognition Act. However, and this is the crucial point, if a man changes his legal gender to female it does not give him access to female spaces.
There is a myth that with self-identification a man would be allowed to say that he is a women - say on Friday afternoons. However, nothing could be further from the truth. If the process in Ireland is followed a legal instrument - a statutory declaration - has to be filled out in front of a solicitor or other suitably qualified person. And the wording has to say he has a "settled and solemn intention to live in the preferred gender of female for the rest of [his] life". And lying on a statutory declaration in the UK is a criminal offence - punishable with up to 2 years in prison under the Perjury Act.
If a man were to change to his legal gender to female and sneak his way into the women's changing room it would still not prevent him from committing the offences of Voyeurism and Exposure. Both of these offences are drafted in gender neutral terms.
Not only is the Man Friday campaign unnecessary, it is positively harmful. The constant stream of articles in the media saying that men can identify as women on a Friday and use the women's changing rooms have two effects. Firstly, people who work in leisure centres believe this to be the law and stop challenging creepy guys trying to access the women's changing room. Secondly, it gives perverts the idea that they have the legal right to perform perverted acts. (they don't and these acts remain criminal offences)
And this is not just a theoretical analysis - this actually happened in Seattle, Washington. After a prolonged Republican media campaign stating that a new law allowed perverts to access the women's changing room, a man took them at their word and, claiming rights under the new law, decided to change with the women. And the sad thing is that the staff confused by the media coverage did nothing to stop him. But the kicker is that the new law did not allow a man to self-id his way into the women's changing room. And this is not my legal analysis - this is the legal analysis of the Washington State Human Rights Commission.
So in summary women's spaces are not threatened by either the current law or the government's proposed changes. However, if the Man Friday campaign plants the idea into people's minds that men can self-id their way into women's changing rooms, then expect to see perverts take them up on the idea.