Granted there might have been many discussions on these lines, but there is always the possibility of a new perspective isn't there?
My theory is built on the premise that some of our chromosome didn't quite decide which side of the gender fence they want to sit on. If completely male and completely female are 2 ends of the spectrum then the entire TG/TV/TS/CD community falls inbetween. To draw you a picture, I see it as a scale of evolution/development for the chromosomes. While for some they develop fully resting firmly on either end of the spectrum for others (us) they stop at various points. There are those that are happy simply wearing the clothes every now and then with no desire to pay attention to the appearance while there are those who want SRS. In other words, we are CD's or TG's (or whatever you want to call us) simply because of the same reason that some of us are TS's, except not to that extent. If there is a clear theory on why people are TS then I suggest that it's the same reason that we are CD/TV/TG.
Make sense?