If you wonder why anyone would object to “Cis” (and doing so does not make that person a TERF, by the way, it’s because people want to be able to decide what to call themselves. Women were fine using the term women since time immemorial. Because a group came up with a term 15 years ago for what they want to call another group means that other group didn’t choose the term. If you want to be call woman or trans woman or non-binary or anything else, it is up to you what you are called. Why wouldn’t it be up to women who were born with female biological sex to decide what we call ourselves? How is it ever acceptable for one group to tell another what they are called?