Why do I need to answer questions about my anatomy?

The I-Ching does not judge. It simply represent the current moment. Tomorrow you might have a different result. Each line simply represents what is so, in the  present moment.

Why does my Hexagram have a line with an X and/or an O, and doesn’t look like one of the 64 Hexagrams?

The 64 hexagrams only reflect static lines. These would be people that have one firm answer to each question.

If your answer was in-between then you are in a “change period” between one state to the next. that period is represented by a “changing line” (X or O)

The total possible combinations of all lines (changing and static) is 1296.

How long does a “change period” last?

As long as it needs to.
If you feel like your gender is more one than other, this feeling can last your whole life.  This feeling can change over time and your statement would be only one of the two, which indicates that the “change period” is over, and the new state will last for quite some time.

An external aspect such as our physique or our fashion expression (lines 3 and 6 of the hexagram) the shifts between one and the next can happen much more rapidly than the one of our deepest aspects (lines 1 and 4 in the hexagram).

Why do we need this system? I already know gender is fluid…
The “Gender Code” model combines aspects of body, mind (biology and gender), as well as time, to represent the complete person.
The symbols that represent you in the present moment illustrate the complexity of the components that make up who you are.
It breaks down the nuances of gender and defines each unique stage. It creates a language – well… symbols as a start. When you have a symbol that represents you, your gender and your body, you are visible.