Ok, I’m going to assume you’re a good faith asker here, and will attempt to respond to the spirit of you question.
The law, as it was written, and as we now see from the SC judgement, made it clear that there were certain protected characteristics, upon which no one should be discriminated against.
So, under normal circumstances, you couldn’t advertise a job as being only for men, or pay a gay person less for a job, or turn a person who had undergone gender reassignment away from a hotel.
But it was also recognised that in some
circumstances, we, as a society, do have to discriminate — for the safety and privacy of women, the law therefore allows single sex exceptions. These tend to be toilets, changing rooms, rape crisis centres, etc.
I’m going to skip a lot of stuff in the middle about people’s interpretation of the GRA, because the main thing that trans people and their allies are upset about is the following:
Stonewall, who have been embedded in almost every workplace for many years, have pushed policies that say that all trans people must be allowed to use the spaces and services that align with how they feel. So if they are a natal man who feels like a woman they must be allowed to use single-sex women-only facilities. Stonewall (and other groups) sometimes (if they are explicit about it) call this “getting ahead of the law” — that is, they believed, or said they believed, that it would only be a matter of time before the law would catch up with them. They exhorted all their partners to follow this policy, and to punish anyone (employees, clients, the general public) who objected.
So for 10-15-ish years, trans people (and many others, frankly) have thought, due to Stonewall’s interventions, that they had a legal right to use facilities that aligned with their feelings not their sex, and to call people (women in particular) bigots if they objected because this by definition makes single-sex facilities mixed-sex facilities. But crucially, most of these people using or supporting the use of opposite-sex facilities thought they were in the right with regard to the law. Because Stonewall told them, and they are supposed to be the experts.
The SC judgement, which again did not make new law, only made clear what the law was, has specified that when the law says woman or man, the law means sex - biological sex only. This punches a great big hole in Stonewall’s “getting ahead of the law” policies. But it also means that all those people who thought they were obeying the law because Stonewall said they were, think that somehow something new has come in to “take away” their right to use or support the use of opposite sex facilities.
They think they are being deprived of something they thought they could do, by some new ruling. They don’t understand that they shouldn’t have been allowed to do it in the first place. The people who tried to get ahead of the law, in fact broke the law, and in doing so gave a whole load of now unhappy people the idea that they were in the right, when they were not.