But we refer to people as gay (or whatever the appropriate term) and not homo unless the intent is to insult. As far as I'm aware cis doesn't have negative connotations and I've only ever seen it used to clarify conversations about trans issues. I've never heard it used in everyday conversations where the norm is to use the pronoun of everyone's gender identity (he/she) unless someone requests you to use they.
My personal impression is that the term is often used in inflammatory context (but I guess it's because most often I read it in this thread from an inflammatory tweet). Even if the term is not intended to be insulting, the whole context can lead to people resisting the word.
Isn't there a term in english "Offense isn't given, but taken". Neither terms coloured nor negro were considered offensive at the time, actually they were the polite versions ( at least I believe so, I'm neither american nor old enough to personally verify, but I get this impression from period movies, which is a flawed source in itself). I don't believe someone would use those words to offend, yet they are still considered highly offensive. Remember Cavani and Negrito incident?
If people take offense at the term, isn't it politacally correct not to label them as such?
So basically a new, ugly sounding word, used to describe people that consider themselves normal and healthy, mostly used by their political opponents and people that the first group considers suffering from mental illness or medical condition. As the word is often used by minority people that are often abused by the first group it's no wonder their usage (particularly the most public/viral one) of the word will be in inflammatory context.
I think also the issue is, the group that doesn't like the term, considers them healthy and normal and want to be referred as that. They consider trans people as having mental illness or at least medical condition. I believe they take offense at the term as it implies an equivalence with trans people, meaning by accepting the term it means their gender is also a condition.
I guess if you would go around and talk to your average Joe and call him cis, he'd first not know the term and after explainig he would say: "I ain't any of that cis BS. I'm normal!"
Now, what they actually mean is that they are standard, but there is a whole philosophical and linguistic discussion between what is normal and what is standard behaviour/state/condition.
I'm not at all surprised that the term is controversial. It's possible it will become normal with time as it gets adopted with younger generations. It's also possible it will remain controversial and dependant on political affiliations, but it's possible that it will be considered an insult.
English is not my first language, but I hope I'm managing to explain my self.
