The Historical Jesus

[Ferrocyanide]

I am claiming that a person might have a female brain and be inside male body (the brain is part of the body but I am not including the brain).

And I am saying that even if a person has a "female brain" (whatever that is) it would not be determinative of that person's gender. 

Because brains are anatomical organs, and we no longer believe that gender is determined by anatomy. 

Nor is gender determined by preferences, like preferring to play with trucks. Because females may enjoy playing with trucks. 

The only thing that determines a person's gender is how that person identifies.

It sounds like you are saying that gender is based on an individual claiming to be male or female with no reason.
In that case, the words male and female lose meaning.

I would say that gender is primarily based on sex (the outward appearance). If humans did not have a different outward appearance, if we just reproduced via cloning or maybe we didn’t reproduce at all and we lived forever, then the words male and female would never have been invented. Some people might want to play with GI Joes and some might want to play with Barbie, but we would not use those as a distinguishing feature.

So, most people are normal. The human with the outward appearance of a male likes doing certain things. He grows up and is attracted to the female body.
The male and female get married and have kids.

Everyone else goes into some other category.
Trans people are evidence that “normals” do not compose 100% of the population. There is some defect. For some reason, either through genetics or due to nurture, they are different. The trans observes females and he feels like he wants to have a female body. He already likes playing with Barbie, dressing her up, brushing her hair.
So, it is not a random choice that the trans is making. The trans person evaluates the humans around him and decides to be in the female category.
I would say that he has a female brain.

Yes, the difference between male and female is not so clear cut. Some decide that they don’t fit in in either category: male or female.

****All the above are my guesses. I don’t know enough about gays, lesbians, trans people.