Does your gender or sex define you?

[Cavernio]

"You're making another uncritical assumption here, and that is that heterosexuality is a product of biology where homosexuality is not."

No, I'm not saying that at all. You can't take what I say for myself to mean that I think that everyone feels that way, or turn that into an assumption of what homosexuality is. I doubt a biological change late in life would have the same effect that biology in the womb has. For instance, we alter the biology of men or women to change their sex, but do we even have any success of changing someone from straight to gay, or a much more sought after practice, gay to straight? Even if at some point someone did manage to actually change someone from gay to straight, it would likely be a result of brain-washing and not biological changes.
On another note though, I've a womean who specifically said "I'm done with men, all the ones I've known have hurt me", and if they never considered themselves lesbian before, at least act like they are now, and want to be considered that way. I in fact strongly believe homosexuality is a result of biology. I assume that TG is as well, although I have not really read as much about it as I have homosexuality. I will not ignore the social part of our genders and sexuality though, nor ignore that what we do may not be congruent with what we actually feel. All that said, which sex I would like after a sex change is really a side-note to what I feel like I would be.

"It would be like if I made a poll asking if the reader had hypothetically been born without fingers on one hand, and with fingers on another, would this bother them? Everyone who voted yes on that would be dismissing your own feelings as invalid, and they would be doing it without having even the slightest experience with the subject."

Firstly, I do find areas in my life where I would like fingers, and hence even if it feels normal, it doesn't mean I don't want them. Secondly, regardless of that first sentence in the paragraph and all the implications it may or may not have that I haven't thought about yet, and on a much more general note, what someone else feels doesn't invalidate what I feel, nor does it work the other way around. IMO that's a life lesson everyone should learn. It acknowledges the fact that we are individuals who may not fully understand one another. It also stops people from getting hurt, while still allowing people to actually have opinions about things. If we don't do that, then we're saying that only guru's should have opinions about things. Better to have an opinion than basically be brain-dead about something, or to just agree with someone because they're the expert. We all are assigned a sex and have a gender, we all have some experience in this.
I think Rubin0's experience of not wanting to dress and act feminine, while still not saying 'I want a penis and balls to go with that' shows that not everyone who feels like they're not their gender feels like they're in the wrong body...an experience it seems you could never have.