Good morning, DK.
I'd just like to point something out: the duality issue you keep focusing on is actually irrelevant to the piece.
Of course, perhaps you will say: "No, it's not." But consider...
By all accounts, Nigel's included, the general tenor of the piece is about gender roles and identification. You must agree that the author would be one of the better sources of information about what the piece deals with.
So, what happens if we completely strip the piece of dualism?
"Oh ho," you might say, "I have demonstrated clearly that the dualism exists implicitly throughout the piece. You cannot strip away what I have concluded is the basis of the argument."
Ah, but. Ah, but. What if we were not purely in the realm of logical argument? What if we changed the game rules? After all, this piece exists as it is, there are no tags attached to it proclaiming it to be a 5-paragraph persuasive argument; as an intelligent Discordian, I presume you are fully aware of the Law of Fives, both the written and unwritten sides; thus, I'm sure you can agree that we, as the reader, can use whatever Game Rules we choose. Therefore, I propose we put the piece into a Between realm, as such:
LET US CONSIDER THE USE OF DUALITY TO BE A POETIC METAPHOR.
So, now we can agree that while the Cartesian Duality is roundly considered to be flawed and unwieldy, we can also agree that many, many (perhaps even a wide majority; so much so that the minority may be statistically insignificant) people have felt at one time or another that their bodies sometime seem to act of it's own accord (I'm sure with your intellect and knowledge I don't have to go through the old examples and arguments).
You see, even though the duality may have been disproved, the appearance of duality does indeed exist in the experiential world. And because of that, we can create a metaphor using it. And because it has become a metaphor, it is no longer considered as a "premise" in this "argument".
That being said, let us look at the piece again:
Paragraph 1:
A) Humans usually come in Male and Female.
{comment: Simple enough, I'm sure we can all agree on this.}
Paragraph 2:
A) Humans make arbitrary definitions about the aesthetic appearance of their bodies, which they then use as a factor of how the perceive themselves.
B) They also do the same with sexual behavior, and with gender roles.
{comment: The evidence here is observational; it is clear that the author does not wish to be pedantic, nor does she wish to create a tangential aside as to the reasons why humans tend to do this, as it does not seem pertinent to the essay in the specific.}
Paragraph 3: (I do not consider the one-sentence line a paragraph; I consider it a literary device.)
A) Humans tend to forget that while the body's chemical processes occasionally affect behavior, most of the arbitrary definitions they create are indeed arbitrary.
B) If all arbitrary definitions were removed completely, humans would still exist.
{comment: Again, the evidence is observational. True, some readers have to make more of a stretch to see this, but it does follow from the previous paragraph.}
Paragraph 4:
A) It is unwise to attach your perception of self to arbitrary concepts and categories, due to their arbitrary nature.
Well, there you go. The intent of the essay holds, even after removing your main objection; therefore, Cartesian Dualism is not the premise of this piece.
I hope that with your keen intellect you can follow my humble offering.
