The term cis was originally used only in a strictly medical context in which a simple and easy way was sought to differentiate those who identify as trans from those who do not.
Volkmar Sigusch, a German psychiatrist and sexologost coined the term "Zissexuell" (i.e. cissexual) in 1991 because he reasoned that if there are transsexuals, there must of necessity also be cissexuals.
Interestingly, Sigusch's work on transsexualism includes such gems as categorically stating that no male person can ever understand what it means to be female (and vice versa).
In his understanding of the ideas behind transsexual and cissexual (he only coined the word cisgender in 1995), it's all very much about the absence or presence of dysphoria. And about the presumption that it is healthy to accept the sex stereotypes and sex role stereotypes associated with one's sex.
Following his invention of these two words, they were rapidly adopted by those working with patients who suffered from gender dysphoria. In their writing, it denotes those who are not suffering from gender dysphoria and who accept the sex stereotypes and sex role stereotypes associated with their own sex. This is framed as the healthy, the natural response to stereotypes.
If you have read any second wave feminism, you will know that even at the time that Sigusch coined the word "cisgender" to denote the healthy and natural response to sex stereotypes, there was already a large body of feminist work that explored and explained why it was natural and healthy for those of the female sex in particular not to embrace sex stereotypes and sex role stereotypes. Because these stereotypes were used to oppress women and girls, to discriminate against them, to deny them equal access to opportunities.
So from the outset, the concept of being "cisgender" as the natural and healthy state of being conflicted with the feminist understanding of sex stereotypes and sex role stereotypes as being particularly damaging for the female sex, regardless of whether these stereotypes were willingly accepted by those they were imposed on or whether they were resisted.
Now it is entirely possible that you do indeed embrace these stereotypes and happily embody them. But I would bet money that there are some preferences, personality traits or skills and interests you have that are not stereotypically associated with the female sex. Because few of us embrace all of the stereotypes associated with our sex. Of course, far more people than we can count embody these stereotypes without embracing them. That's because we are socialised into them and it is very hard to shrug of one's conditioning. It is even harder to defy societal expectations when that risks exclusion, censure or punishment. The word "cisgender" does not allow for partial rejection of those stereotypes, nor does it differentiate between those who happily embrace the stereotypes and those who are forced into them.
So that's one reason why imposing the label "cisgender" on others outwith the strictly medical context in which it was coined is an issue - it is simply incorrect for the vast majority of us.
Furthermore and this is probably more important for us here on the Feminism and Women's Rights board, I should add that in modern usage, the word "cis" is no longer a mere reference to someone who embraces the straitjacket of stereotypes associated with one or the other sex. It is also and especially posited by many trans rights activists as denoting the oppressor class in a proposed oppressive system whose axis is gender identity. That's why "cis" isn't just offensive to me, it's gynephobic.
That's because there can be no axis of oppression based on how society reacts to a given individual's preference for the sex stereotypes and sex role stereotypes associated with the sexes. Just as with outright oppression on the basis of sex, the correct axis of oppression for this issue is also sex. And the oppressor class in that system is the male sex class. Labelling most members of the female sex class as cis and therefore as the oppressors in power over those who identify as trans seeks to obscure that fact.