I think the judgement was based on whether or not DrU was in the changing room with permission, and the way in which SP challenged him.
Obviously we don't agree with the judgement or the judge, but he used quite a few of his 300+ pages explaining why he believed that DrU had his employer's permission to be in the CR, and SP overstepped the mark in expressing her GC beliefs, and he quoted a lorryoload of cases to illustrate his interpretation.
1013. What was to happen about the use of the changing room was not a matter for decision by the second respondent. Nor was it a matter for decision by the claimant. It was a matter for decision by the first respondent, subject to the constraints of the law. In our view, Article 8 of the Convention gives protection to the personal sphere of each individual, including the right to establish details of their identity as individual human being. That applies both to the claimant and the second respondent.
He also went into detail about the protected characteristic of gender reassignment, and how DrU was entitled to it, and that it is not based on 'looking feminine'. He specifically mentions that transgender people may retain the appearance of their original sex, and still be legally entitled to the PC of gender reassignment. He also acknowledged that some people think that DrU looks like a man.
898. The person may present fully in the gender after transition, or to an extent in the gender desired. If the latter that may be where most or some people looking at the person would be able to recognise aspects of physiological attributes of the sex as assigned at birth. There is not always a simple answer to the issue of appearance. Some such as the claimant regarded the second respondent from what she observed as male. Others who may observe the second respondent may regard the second respondent as a female....
I think the knots he tied himself up in trying to pin down when the right to protection under 'gender reassignment' kicks in, taking into consideration the past, present and future status of the 'transition':
942. We also consider that the terms of section 7 are instructive. Whilst sex is binary, as we noted above, the position of someone who has the protected characteristic of gender reassignment is not. We also noted above that there are three tenses used in the section – the future (proposing to undergo), the present (is undergoing) and the past (has undergone). That suggested to us in the present context that the stage at which the transition has reached, assuming it has commenced, is another relevant factor.
illustrates that legislating for things as vague as 'gender' makes the law look silly.