The gender binary says that there are only two genders—male and female—and most of the time this model of gender excludes transgender people also. However, in most feminists minds this is not the case, and the model allows for fluidity between two genders. This is still not enough though. Many people in the world identify as a gender or lack of that does not fit this model at all. Its easy to dismiss these people and say that they are new, and that they weren’t around until the past century or so, but is my belief that these people always existed, they just weren’t given the language to explain their (a)gender identity. Intersex people occur every 1 in 1500-2000 births, so thats actual biological proof that people can be born quite obviously nonconforming to the gender binary. Unfortunately, most doctors do unnecessary surgeries to do what they perceive as “fixing” these people, which can be incredibly psychologically harming. There are agender, gender fluid, bigender, genderqueer, intersex etc. people in the world and time and time again they are silenced, whether it is by their parents or institutionalized silence, like only listing two genders on a form or survey or the assumption that all people with penises are male and that only women menstruate. (Disclaimer: I don’t claim to have never made these mistakes before in my life because as a cisfemale I was born making these type of assumptions so I need to continuously work to get rid of them).
The most basic definition of feminism is to gain/create equality for all genders or lack thereof, and in order to do so we need to get rid of the gender binary and recognize that these genders even exist in the first place. So, yes I do believe that getting rid of the gender binary absolutely 100% belongs with feminism.