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I don't understand the first question. Is this question asking why people want to categorize trans people as strictly male or female?

In response to the other two questions:
Right now I'm thinking that as children most of us are raised to think of people as exactly female or exactly male without strictly defining either term, while also having engrained that this is 'naturally' or 'biologically' (read 'easily') determined and is a fundamental (and so unchangeable) part of each person. Somehow we incorporate this into ourselves so much so that when confronted with someone who says I was born one and converted to the other we have to rethink the way we think about the world. Some people find this much more upsetting than others. At least, this is how I think about this right now. Passing is important to be safe from some people who get so upset about their upset-world-view that they hurt people to control their world view. Why might passing be dangerous? Well, it is a lie. I think lies are inherently dangerous, they allow others to think something that isn't true. This stops them from growing and having a better understanding of the world. I think that this passage from 'Toward A Theory of Gender' is relevant to these questions:

"As long as the categories 'female' and 'male' present themselves to people in everyday life as external, objective, dichotomous, physical facts [it will be] difficult to avoid evaluating one in relation to the other, a firm foundation for discrimination and oppression. Unless and until gender, in all of its manifestations including the physical, is seen as a social construction, action that will radically change our incorrigible propositions cannot occur. People must be confronted with the reality of other possibilities, as well as the possiblity of other realities."