VioletSky
Namsnanny hmm well
Seeing as I'm awake and alone with my thoughts.
Many on this thread won't agree with me on what constitutes transphobic language, especially when it comes to protected beliefs and at the moment it is a bit difficult to navigate but it likely won't stay that way. There are scenarios we can look at that give hints of where it is headed.
Scenarios that are starting to run parallel with clamping down on racism and homophobia.
Hate crime
Transgender people are protected against hate crime. A hate crime is if the offender has demonstrated hostility based on transgender identity during an assault or been motivated to assault by hostility to transgender identity. So if a person were to assault a trans woman shouting that they are biologically a man and can't change sex, that would likely be classified as a hate crime as the crime is shown to have occured due to the victims trans status.
Trans people are afforded protections against discrimination under the equality act.
So work places cannot discriminate against trans people, and they also cannot create a hostile environment. So if a trans person were coming to work or school and facing someone staying "you can't change sex, you are a man" the employee or student can argue that creates a hostile environment and the organisation would need to investigate.
Many work places, organisations and platforms have policies in place for inclusive language and set their own rules for what language is considered hostile or transphobic. For work places this also includes employees social media and how they conduct themselves out of work. So if a worker is saying "you can't change sex, you are a man" this may violate their policies on trans inclusion.
We have historically seen many people lose their jobs after using racist or homophobic language. They won't be arrested for it but there are real world consequences. The same may already be happening in regards to transphobia and if it isn't, it likely will soon.
Then if we look at families and friendship groups. What members of family and friendship groups classify as transphobia also matters and that is another way that real life consequences may be implemented simply by saying that a trans women is a man.
That's my thoughts on it anyway, others will have a different perspective but I think it's a wait and see in 20 years or so like other issues
VS... further to your 'late' night (early morning!) thoughts:
If, in the workplace, one employee accosts another - apropos of nothing - and informs them that as a trans woman they cannot be a woman - that can be taken as an inflammatory move. If, on the other hand, said employees are sitting in the lunch room having a discussion on the matter, and the same individual debates the point and says they don't believe that a biological male, identifying as a woman, can be a woman, that is an entirely different scenario. That is free-speech, not hate-speech. If however a rider is added to the observation, one that threatens or intimidates, that is hate speech, and could be considered to be transphobic - merely stating that you don't accept or believe a biological male can be a woman in the course of conversation, is not.
If, out of the workplace I am photo'd carrying a banner which says that trans gender people should be de-capitated / beaten - or have any other form of violence perpetrated against them - my employee could rightly haul me up for it. If, on the other hand, he / she reads a comment I've made on social media to the effect that I don't believe a biological male can be a woman, in a chain / thread discussing the issue, then my employee most definitely does not have the right to sack me. And if he / she does, then we are on a very slidey slope towards the end of free-speech, a move being dictated at the moment I believe by a minority against the majority - and that majority also includes other trans gender individuals too.