There are a couple of significant issues with the argument that these male people have always used the female single sex spaces. These are in no order, by the way.
Firstly, just because some people decided female single sex spaces should become mixed sex and that this special group of male people could access the space, didn't make it the right decision for society. There was no consultation, just the decision made and female people had to put up with it.
Then, the argument is falsely premised that it was 'just a few' so female people shouldn't mind. And that action then was used to leverage in more and more male people so this 'just a few' argument is used to disguise a slippery slope issue. We are also seeing it used as we speak for leveraging in some male people into female sport.
Then, there is an inherent element of reward for effort either directly stated or indirectly implied with 'those male people who 'pass' / made great effort to 'pass'/ had surgeries etc had access or should have access. No male person who has a transgender identity should be accessing the female single sex spaces at all. Extreme body modifications do not make a male person female, and no amount of body modifications should be needed to access a space that is not and never was for them. It is not a reward to be gained by any level of effort at all. And any person declaring that if a male person has had surgery they can enter, applies pressure too to go that step further to 'earn' being considered 'female'. When this is just not possible and those male people should simply accept that they should not access those spaces and find their own solution that doesn't mean accessing spaces that are female only.
This argument about 'passing' also resolves around the 'you cannot tell' argument as we have seen on this thread already.
Then there is the argument that there is danger to male people in the male spaces because of their presentation as a female person. Yet, we also know that there are some male people with transgender identities who do use the male toilets without an issue. Even so, if there IS an issue with safety for any vulnerable male people in male toilets, where are the campaigns to fix this societal issue? Where are the programmes designed to make male single sex spaces safe and welcoming to all male people? There are quite a large number of groups of male people who are similarly vulnerable to come to harm in male single sex spaces. Should ALL those male people also have access to the female single sex spaces because they face what really seems to be the same risk to their safety? Why is this group of male people so special? Female people, it can be surmised, are just human shields.
There really is an issue too with the argument 'some male people really are transgender and should be allowed into the female single sex space', Who arbitrates which male people 'pass' and which ones don't? Who arbitrates which male people are 'really transgender' and which male people are not 'really transgender'.
Then there is the argument about mental health of those who are excluded. But again, this puts female people in the position of mental health support for a group of male people. Again, without consent. There are plenty of things in life that people are legitimately excluded from. If a person is legitimately excluded, then that needs to be accepted.
Of course, not all male people are going to cause female people harm to female people. Still, using safeguarding risk assessment on a collective group basis, male people continue to be excluded from single sex spaces. Yet, some people argue that male people with transgender identities should not be excluded and should receive special treatment for having a transgender identity (ie. a philosophical belief is the only commonality between those people with transgender identities).
It defies strong safeguarding principles. Because there is no evidence at all that in the UK a male person at any stage of transition is less of a risk of committing a sex offence than the rest of the UK male population. There is certainly no evidence at all that that group of male people have the same or less risk of committing a sex offence than a female person. So, there is simply no basis for them to be separated out for special treatment.
It is irrelevant that some male people historically access female single sex spaces. Because those male people should never have been there in the first place. There was no and still is no argument to support their presence.