Ive been mulling over the free speech arguments around this, and trying to define what I think is different about cis vs naming biological sex.
I agree, it's not about the 'right to not be offended ' (which doesn't exist in any case), its about manipulation of language in order to normalise a particular narrative. Its use is therefore actively damaging some other groups in society ie women, its not simply a neutral label.
As a pp said, whether or not you agree with a particular definition of woman or man, biological sex does exist and is observable and measurable. This I think makes it a fundamentally different concept to a contested belief. We can argue over what words should be used to describe male and female these days, but I can't see a good, logical argument for doing away with being able to describe those biological groups- because they exist and sometimes we need to be able to talk about it (for healthcare, women's rights, etc). It's not like the only purpose of correctly sexing someone is to piss off trans people - often its necessary for safety. Cis is not necessary in this way, its a largely redundant signifier of belief (if you need to specify in an unambiguous way due to the current mangling of language, you could say male bodied or not trans) .
Of course people have the right to describe the world as they see it, but I think this right ends when you are specifically describing others who don't share your world view rather than just describing abstract concepts.
I think the analogy between christians and satanists / atheists is a good one. Insisting that someone is 'cis' or must have a gender identity is like asking an atheist which God they don't believe in, or insisting on calling atheists 'heathens' or satanists - both religious terms. I would hope that most if not all religious people at least recognise that atheists exist (even if they think they're wrong).
The issue for me with cis is that it's being applied to people who don't accept that label. If one group in society has the right to define themselves as something, in this case trans, surely that right also extends to everyone else? Why isn't forcing people to accept 'cis' just as bad as misgendering? It seems to be giving additional rights to trans people at the expense of women, and I don't see why we should accept that.
It's not about authoritarian policing of language to avoid offence, its more complex than that.
Edit for typo