Exposure offence
Difficult to prove in a mixed changing facility.
to intentionally expose their genitals and intend that someone will see them and be caused alarm or distress (section 66, Sexual Offences Act 2003)
A male user would just say that they didn't mean to cause alarm or distress, and didn't mean for anyone to see him.
There is a high bar to the (rare) use of "Outraging Public Decency". (This would be the appropriate offence for watching porn in a public place or within the view of a member of the public - but not indecent exposure.)
If you design a mixed sex space for people to get undressed right down to their birthday suits, then exhibitionists have got a "get out of jail free" card. In effect, the management are declaring "caveat emptor" if you want to use their services.