1. There are only two cis genders— male and female/man and woman (which are seen as being the exact same thing pretty much). Being cis thus necessitates by definition buying into a binary gender cistem— err, system.
2. Being cis depends upon accepting the gender you were assigned at birth. Birth assignment is an inherently binarist and frequently violent practice because it involves A) insuring that all bodies conform to one of two acceptable body types (“sexes”), resulting in the mutilation of many intersex infants, and B) legally designating every child as either male or female, seriously inhibiting that child’s ability to express, experiment, or play with non-conforming gender until and in fact into adulthood.
3. Cis people benefit from cissexism, the system of oppression which privileges cis people over trans people, and from its subset binarism, which is the belief that only two genders can be real and natural. As such, cis people, whether they realize it or not, have a vested interest in upholding cissexism and binarism.
4. The majority of cissexist and binarist oppression is perpetuated by cis people, ranging from transphobic murders to transphobic physical and verbal harassment to transphobic job discrimination to upholding of transphobic and binarist standards of appearance, dress and beauty to upholding the continued existence of gendered bathrooms (in most cases opposing or declining to provide even a gender neutral bathroom option alongside gendered bathrooms).
How being trans reinforces the gender binary:
3. It doesn’t.