April 6, 2017 - The Rise and Fall of the Ex-Gay Movement
A little over a century ago, the nascent science of psychology took an interest in the origin of same-sex desire. Up until that point homosexual sex was seen as a vice to which a person might be habituated for any number of reasons. Psychologists, however, construed it as a mental disorder which had roots in early childhood development. Both the notion of a sexual orientation itself, and the idea that sexual orientation could be altered by treating childhood trauma, originally derived from this foundational supposition.
These new trends in the research, coupled with changing social mores and pressure from gay-rights organizations, caused the APA to remove homosexuality from the Diagnostic Standards Manual in 1973. Being gay was no longer seen as a mental illness.
Except in Christian circles.
See the whole article on Patheos