What is a 'gender identity'? Please give us an accurate and non-circular definition of what each gender identity is and how people know they have one ?
For instance:
No male person can ever live or experience life as a 'woman' or a 'girl'.
They can only ever experience life as a male person who believes they are a woman.
Even when they 'act' like a woman, they are acting as they believe a 'woman' should act. Which is fucking misogynistic when you unpick it!
Even if they are treated 'as a woman' by some people, they are being treated as a 'male who presents as a woman and believes they are a woman'. Because their every reaction is based on that. Not on them being female in any way.
But based on their own reaction developed over years of them reacting to the way they interact with society and themselves and how others interact with them while having a male body and a mind that processes stimuli in that male body.
Even when they have extreme body modifications, it is to be their own concept of what a female looks like to them. It is not what a female is.
They are still processing stimuli and making decisions based on the way they interact with society and themselves and how others interact with them while having a male body.
How can it be any other way?
The only way a person can experience life as a woman, is to have a female body, formed around the production of large gametes, even if it doesn't produce those and to navigate their life based on the decisions they and society makes that revolve around them having that body.
A male can conceptualise what it might be like to be a female, but that is all it ever is - their concept of being female.
They may do it because they don't feel they fit into how they conceptualise how a male person interacts with the world (ie. their own stereotypes around being male) or they do it because they want to be seen as a female (using their own stereotypes of how a female navigates life). It really doesn't matter though. Their motivation is irrelevant to the outcome. And I consider the outcome can only be described as misogyny.
Which is that they will always be just a male who believes they are something they are objectively not. Or who doesn't what to the thing they objectively are.
How can the material reality be any different? This is why someone's gender is only based on someone's philosophical belief. And philosophical beliefs are fine for people to hold, but not one person in the UK has to comply with another's philosophical belief.
The logic cannot be any different than that I am afraid. But apparently, we should just let these male people use female language and into female single sex spaces.
You can choose to use the word 'cis' for yourself, and to use whatever language you want to for others. However, it is in no way harmless to make these language changes when you start to understand the collective harm that using this language can result it.