Why we in the TG community feel a need to argue about something so trivial is beyond me. However, I have my personal opinion as well, so here it is.
I am quite happy with my gender - that is, "feminine." I am not happy with the sex that the doctor assigned me at birth - that is, "male."
All my legal identification papers, primarily my driver's license, list me as "male." They are listing my sex, not my gender (regardless of what the label in the application form says). To a large extent the legal stuff ignores my gender.
So what I want to change is all that legal identification that, to me at least, is wrong. Therefore, what I want is my sex reassigned.