Earlier quoted context omitted.
The only people who would have cis used to "delegitimize" them would be ones who are saying something innacurate about trans people and are being told that they do not have relevant frame of reference for that comment. Describing cis as a slur is... Wow.
It was created for a couple of purposes: - Add an unnecessarily complex psuedo-intellectual layer to the discussion in order to catch normal people off guard and make them feel "uneducated" so that they can be "educated". - Decouple the idea of normal from heterosexuality, which is, frankly, normal due to it being the most common state for of existence for people. Overall it is a purposeful attack on the status quo.…
Um, no, other than arguably being wrong on the broad kind of motivation in either case, you seem to have also confused the word “cisgender” (or “cis” for short) with the word “heterosexual”.
Transgender people can be heterosexual or homosexual or asexual or any other sexuality, and the same is true of cisgender people; having a term to refer to people who do not deviate from the until-recently-in-our-society culturally obligatory stereotypical relation of gender identity and sex characteristics at birth does nothing to reduce heteronormativity of language compared to just referring to them as “normal” (and then, depending on context, elaborating further on the axis of normality and possibly whose norms are being referenced), to the extent that having a word does that, its the word for people who don’t deviate from the until-recently-in-our-society culturally obligatory stereotype of sexual preference for partner gender based on their own gender.
But, independent of undermining the norms involved, its sometimes useful to be have a convenient concise term, which both “cisgender” and “heterosexual”, in their respective domains, provide.