I think there's a few differences between 'heterosexual' and 'cis', however. I consider myself straight, but I wouldn't actually have to label myself as heterosexual. No one has ever insisted on it, in my experience, and if you refuse the label, the assumption is that you could be homosexual, bisexual, or asexual. Also, these are labels that describe a relevant part of myself and my identity, as they do for most people - who we are sexually attracted to.

But cis. For one, people DO insist you accept that label if you don't call yourself trans. They are saying that gender identity is a relevant part of yourself, even if you disagree. Well, I don't know what it means to "feel" like a woman or a man. I don't believe gender identity exists. But the 'cis' label is still forced on me, implying that I comply with the sex stereotypes of my birth sex. And yet, I don't. My childhood is filled with memories of hating being a girl, playing with the boys, with boy toys and boy games, and of struggling well into adulthood with being 'ok' as a woman. To the TRAs, none of that matters. My experiences, my feelings, how I see my identity, doesn't matter. I'm cis because I know what biological sex means.

LGB activists don't call you a bigot for 'sexual orientation ambivalence'. So yeah, cis is absolutely a slur.