Psychology is a branch of science, not medicine. The actual definition of psychology is: The scientific study of human behavior and mental processes (Discovering Psychology, 3rd Edition, pg. 3). Psychiatry is a branch of
medicine that deals with mental, emotional, and behavioral disorders. Please do not confuse the two terms. A psychiatrist is enabled to prescribe medicine, a psychologist is not.
The thing about science, Talyubittu, is that ALL of the acknowledged sciences rely on hypotheses and proving/disproving those hypotheses in order to define our world. Psychology is a COMPLETELY legit school of science because it follows the laws of the Scientific Method, using hypotheses about how the mind works to piece together the puzzle of our minds and behaviors.
Your point about homosexuality being a mental disorder 40 years ago is moot. Psychology as a science has analyzed homosexuality many, many times over the years, through thousands of different circumstances and people, and we have reached a conclusion as a school that homosexuality is a legit sexual orientation. That is our modern belief.
You'll recall that recently, science has changed it's collective mind on several ideas we once took for fact. For instance, Pluto is no longer considered a planet of our solar system. The Human Genome project: It was once believed that you had a gene for every single protein in your body. That has since been dis-proven. Finally, transgenderism is now believed to be a biological disorder instead of a mental disorder (
http://www.harrybenjaminsyndrome-info.org). It is classified as an inter-sex condition. (Disclaimer: HBS is classified as Gender Identity Disorder by the DSM-IV TR, but the scientific community is confident that in the next revision of the book, GID will be replaced by HBS.)
Science evolves through the ages, my friend. Keep in mind that, during the early days of human civilization, most civilizations welcomed and embraced homosexuality. Greece, Egypt, Sumeria, India, Rome, etc. Science is now going full circle with it's idea on homosexuality.
As for comparing psychology and pediatrics, I do not think that is a valid comparison. Psychology is an investigational branch of science, a family it shares with biology, chemistry, and physics, while pediatrics is a medicinal science, along with radiology, urology, endocrinology, and oncology.