I understand what you’re saying but don’t think I can agree.
I don’t think differentiation is the same as division. There should be a way to say someone is not trans that does not imply being trans is not “normal” or outside of the mainstream.
I guess we could always just go with “not-trans,” but it is much clunkier than saying cisgender, and a lot of people who are not trans don’t like that it centers the trans experience (non-trans people only being defined by not being trans).
I have asked people before, when they complain about the word cisgender, what they would rather be called. It is usually cisgender women whose complaints I’ve read, often in comments sections like this on Medium. Overwhelmingly, their answer is just “woman.” In other words, they don’t recognize trans women as really being women.
In fact, I have not heard an opposite to transgender proposed that actually validates transgender people as people (calling cis people “normal,” implies we are not normal; calling cis women just women implies we are not women).
So, far from differentiating and dividing us from other women, the word cisgender — as an opposite to transgender — actually validates transgender women as belonging to that larger group that is called women.
I think the problem is that we look at trans or cis as being the only descriptors. I am a trans woman. I am also a white woman, a middle-aged woman, a relatively tall woman (5'9"), a queer woman married to another woman, but often it is ok to just refer to me as just a woman.
And, of course, we don’t have a problem with these other descriptors. If a black woman calls herself a black woman, that would not erase the fact that she is a woman; it just adds more information. It also would not automatically divide those who were not black from her. If a white woman were to take offense at being called white, would she — when asked to be called instead — just say, “just call me a woman?” Probably not.