1. That it's not a word you're comfortable with. Of course, that raises the question of why. But putting that aside for a moment, it is reasonable to ask people not to refer to you with words you're not ok with. The logical consequences of this argument are that a) you need another way of saying explicitly when someone isn't trans (which could just be 'not trans'), and b) you also need to accept that some women are fine with being called cis.
I don't see anyone here saying anything other than that they find the word offensive. No one is requiring that everyone else adhere to their personal choice of labels (other than those who insist women must accept the word cis.) Basic principle: everyone's free to identify themselves based on their own sincerely held beliefs and won't attempt to label others against their will. All good.
2. That 'cis' is unnecessary because women are 'not trans' by default. This is a weaker argument. We often need language to make things clearer; language doesn't develop on the basis of using the minimum possible words, it's for maximising communication. The idea that there is such a thing as a default person is also a hard one to argue in a multicultural society.
Women and TW/NB/TM. The words are there. Insisting on 'cis' is motivated by other reasons than clarity and meaning; and again as you point out, not all women would choose to identify as 'cis'. So we're into 'cis' woman and TW and NB and .... wtf do we call all those people over there because we've ditched the word 'female' and 'woman' that did the job perfectly well until five minutes ago.
3. It's like calling an atheist a heathen. This isn't a good comparison, because atheists aren't proposing that 'human' should mean non-believing-in-god. They're fine being called 'atheist', and they're fine accepting that religion exists even if they don't subscribe to it. If someone wants to propose a word that means 'person who doesn't believe in gender' (which is similar to what Gender Critical means), that's valid but doesn't have the same meaning as cis, especially when there are GC trans people.
This again pre supposes that there is only one correct belief, which is in gender ideology, and those not subscribing to it must find other ways to describe their intransigency. Instead of respecting that many people just don't believe this way of regarding the world and continue to believe in fixed biological sex and to define themselves accordingly.
4. That it incorrectly implies everyone has a gender identity. This argument can go two ways: either that people don't have a perception of their own identity or that gender doesn't exist as a social construct. Generally these arguments are based on a misunderstanding of what identity, gender & social constructs are.
This basically suggests everyone does have a gender identity, just that some remain in denial about it. Again, rejection of accepting that there are many who just don't see the world in this way, don't wish to define themselves in obedience to a political ideology or participate in it, and this is ok , it's possible to live and let live.
5. Cis makes women a subset in their own category. The main problem with this argument is that it misuses 'subset' as it applies to set theory (things cannot be sets, they are contained within sets), and the argument is incomplete because it doesn't describe what harm is supposed from being in a subset. VIPs are a subset of concertgoers, sprinters are a subset of athletes, cars are a subset of vehicles - no harm is being done to people within these sets by describing them as such. Considering the set of women, trans women and cis women are subsets, but so are old women, blonde women, tall women etc.
Thing is, all of those sets are female. The implication is that 'women' is a mix of sexed people in different sets. To those who believe that women is simply the half of the human race who are biologically female, this seems merely a linguistic way to crowbar male people in to their words and spaces. Which advantages male people but disadvantages female people and is good old fashioned sexism.