All the above. Transactists will claim two reasons why you are cis. The first is because you're not trans and it's to differentiate.
But most people object to being defined by something they are not. I'm sure men do not want to be defined as 'not a woman who thinks they're a man'.
The second reason is if your 'gender' aligns with your sex.
This only makes sense because their definition of gender means an innate sense of your sex that does not relate to your biology. So if your innate sense and your biology are the same, you are cis.
Of course, what determines your sex IS your biology. Not your attitude to your biology.
Feminists' definition of gender is a set of societally imposed expectations and stereotypes. So there is no way they want to align with it. Frankly, most people don't align with it anyway.
Interestingly every single official trans organisation will also say that gender is a set of expectations. Because there is nothing to support the idea that it is an innate sense.
The hidden agenda is that transwomen and cis women are both subsets of 'women'.
With the goal being that people end up talking about women and cis women.
You can verify this by asking to not be called cis, and see how you are ignored, told you don't understand it, and you need to suck it up.
Hence most women find it very offensive.