Passing is being seen in your desired side of the gender binary enough that people assume you physically belong on that side of the binary. This goes along with societal gender roles. I actually wrote a pretty long post about this sort of thing. See below
I had gotten into a bit of a discussion on facebook. I had put a lot of work into this reply, but I didn't get much of a response. I guess maybe it was too much to rebuttal. I enjoy a healthy, mutual, and respectful debate of each other's views on things; and I am interested in hearing all of your views on this subject.
I had brought up the topic of passing. Another person said that passing is silly and that gender binaries are "passé" and that she passes as a human and that is all she needs. Well this got me to thinking. So I typed up this.... long winded reply. And here it is.
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The gender binary is believed to be a construct of our modern society. But the fact is it's been around for as long as there has been breathing creatures on this planet. Virtually all forms of sentient life live under this gender binary. Why? Because there are females and there are males. This is a physiological fact that holds true in millions of forms of life.
I'm not talking about Gender Roles, because you and I both know that a lot of traditional gender roles are silly. No, what I'm talking about is our very natural, evolutionarily driven gender binary. Perhaps mentally there is a "third gender" – but as far as nature cares, that third gender might as well never exist. Men use sperm to fertilize a female egg. We procreate, we survive, and we thrive. This "third gender" cannot survive without the natural gender binary of nature.
The sociological aspect of this gender binary makes perfect sense. The reason why women are women and men are men is simple to understand, that is how our evolutionary drive to procreate works. It is much simpler to have distinct, identifiable differences between the two sexes for easier procreation. You can find this everywhere in nature, look up sexual dimorphism. Our gender roles are largely evolutionarily driven.
Men were in charge because men were physically stronger due to testosterone acting as a strong steroid. Only recently in our modern society have women started to gain power and rights. Gender roles change over time and are as diverse as the number of cultures we have represented on earth. If you wish to exist in any society, you must "do as the romans do". You are pretty much required to live by their gender roles.
Why? Disregarding these gender roles in the very best of circumstances, you'll be viewed awkwardly; people may avoid you and without a doubt ridicule you behind your back. Even in the "AMAZING USA" you can be lawfully terminated from your job just for being Trans in a staggering number of states. In other societies you can be jailed and even executed.
While some of the gender roles that we face are silly and completely arbitrary, it's better to follow them and attempt to change them rather than completely disregard them. You'll live more peacefully and the people near you won't be uncomfortable. You can live a safer and more stable life.
I do agree that our traditional gender roles need to change and that is happening. But disregarding them is foolhardy and even unsafe.
I would venture to guess that you probably follow a good chunk of these gender binary specific fundamentals and even the gender roles assigned to these binaries by your society. Things such as taking estrogen, growing breasts and having an otherwise female appearance are basal to the female side of the binary. Now, I bet you also have long hair, maybe wear makeup, and various other things that are all nothing more than gender roles assigned by society to that gender binary. I bet even the name that you've chosen is following that gender role.
You may not like gender roles or your miseducated idea of what gender binaries are, but you sure do follow them pretty well.
The "Third Gender" mentioned above may seem cool. But when our gender roles are based on a nature driven gender binary, there really isn't much of a place for that third gender. So the best we have is to grit our teeth and conform to the gender binaries of our society and work to change what we dislike.