There's two typical arguments against using 'cis', although neither sound like exactly what you want.
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.
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.
Looking at the other arguments in this thread:
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.
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.
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.