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.
“Trans” is not a label for a third gender that one might have instead of and to the exclusion of man or woman, “trans” is (like, and in opposition to, “cis”) relationship between gender identity and “sex”—actually, ascribed gender—assigned at birth; it can coexists with any of man, woman, nonbinary, or other (or, in the agender case, no) gender identity.
Though your not knowing what “trans” means very clearly explains your not understanding the need for “cis”.
No, it is actually an observation of sex - often observed prenatally, then confirmed at birth.
- 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. In summary, "cis" as a term is unnecessary.