This is from Wikipedia but it rather well encapsulates the main arguments against gender disphoria being a mental disorder and instead more steeped in society’s non acceptance of gender variance or conversely as a birth condition.
Gender as a social construction
Gender characteristics are socially constructed and therefore naturally unrelated to biological sex. This perspective often notes that other cultures, particularly historical ones, valued gender roles that would presently suggest homosexuality or transgenderism as normal behavior. Some cultures have three defined genders: male, female, and effeminate male. In Samoa, the fa'afafine, a group of feminine males, are entirely socially accepted. The fa'afafine do not experience any of the stigma or distress typically associated with deviating from a male/female gender role, indicating that the distress that is so frequently associated with GD in a Western context is not caused by the disorder, rather it is a secondary result of social disapproval
GID as a birth defect
This argument is supported by physiological evidence, such as the presence of typically-female patterns of white matter and neuron patterns observed in the brains of male-to-female transsexual and overall longer instances of the androgen receptor gene. One rebuttal to this view is that these markers do not identify every individual who undergoes transition, and that using them to define transsexualism could falsely exclude some people from treatment.
Distress as a consequence of stigma
The DSM-IV-TR diagnostic component of distress is not inherent in the cross-gender identity, rather it is a result of social rejection and discrimination Dr. Darryl Hill insists that GID is not a mental disorder, but rather that the diagnostic criteria reflect psychological distress in children that occurs when parents have trouble relating to their child's gender variance.
One of the interesting things I have observed during my lifetime has been the change in social acceptance of gender variance. I offer as example an individual like Andrej Pejic himself an example of it and, while he seems to feel comfortable with his biological sex, he exhibits extreme femininity. In repeated interviews, he simply states that this is the way he is and that it is society’s problem if people think he is strange. The fact that Andrej is a celebrated model and has been lauded for his ability to be completely himself, shows how far we have come in the last 3 decades.
While I am not proposing that gender disphoria is or is not a mental disorder, it is interesting to note that increasingly there are more and more people being less stigmatized by the idea of revealing their natural penchant to display gender variance and, for at least some of these people, the draw towards physical transition may possibly be reduced through the removal of certain societal pressure to behave or dress in a manner expected for their birth gender.
Since gender variance has existed for as long as mankind has been on this planet, we have seen that in those cultures where it is not as discouraged those so inclined are more comfortable to express themselves and therefore less traumatized and distressed.
Had I felt the way I do now about myself all those years ago, I would have avoided a great deal of repression, secrecy and guilt over something which was clearly now in retrospect not such a big deal.
Indeed as I have come closer and closer to complete self acceptance, the attractiveness of transition has become proportionately reduced because my distress was only over fear that my behaviour was abnormal or disapproved of and not because it was causing me harm.