The best single resource I found is a 2002 episode of This American Life, titled 81 words. If you have an hour to listen, I would encourage you to enjoy the entirety of it. Based on this and a few other references that I will link in, I think I have a better understanding of the history of the change and why it is correct.
Prior to the 1973 change, most psychiatrists based their treatment of homosexuals on the works of a medium size group of experts, among them Edmund Bergler, Irving Bieber, Sigmund Freud, and Charles Socarides, all of whom had a significant flaw in their methodologies. The homosexuals that these doctors studied tended to come from either prisons, those released from the military dishonorably, or men who hated their sexuality enough to seek treatment to change who they are. There were no well-adjusted, normal, gays among those studied.
There were two researchers before 1973 who studied homosexuals with less or different bias. The better known is Alfred Kinsey, who released a report in 1948 that was incredibly controversial. The most controversial aspect was, perhaps, that he found that 37% of men had experimented with sex with other men and is the source for the claim that 10% of the population is homosexual. It is, of course, more complicated. Far more complicated because the two largest groups of participants in the Kinsey studies were prison populations, similar to the studies by Dr. Bergler and the others cited above, and a second group being male prostitutes. Like those who found homosexuality to be a pathological condition, Dr. Kinsey used biased samples, not quite as normal as one might have hoped.
The second researcher was Evelyn Hooker, who wrote a paper in 1957 based on homosexuals who were functioning in ordinary society.
She administered psychological tests to groups of self-identified homosexuals and heterosexuals and asked experts, based on those tests alone, to select the homosexual people. The experiment, which other researchers subsequently repeated, demonstrates that most self-identified homosexuals are no worse in social adjustment than the general population.This experiment was funded by the National Institute of Mental Health and was probably the first study of homosexuals conducted without obvious bias. In blind studies, researchers found an equal percentage, about two-thirds, of homosexuals and heterosexuals to be well-adjusted.
Dr. Hooker and Dr. Kinsey provided the foundational science behind the activists who did disrupt APA meetings in the early 1970s. Disruption tends to alienate, not to attract allies. So, claiming that the change was forced by disruptions alone is absurd on its face. There was also first person testimony from a gay psychiatrist, John Fryer.
Dr Fryer's speech started with the words "I am a homosexual. I am a psychiatrist" and continued to describe the lives of the many gay psychiatrists among the American Psychiatric Association who had to hide their sexuality from their colleagues for fear of discrimination, and from fellow homosexuals owing to the disdain in which the psychiatric profession was held among the gay community.A year after Dr. Fryer spoke under the pseudonym Dr. H. Anonymous, the change was made. Homosexuality was no longer considered an illness. Other medical organizations studied the change in policy at the APA and made changes to their policies over the following twenty years.
81 words, which I cited near the top, is by the granddaughter of John Spiegel, who was president-elect of the APA at the time of the change. His granddaughter, Alix Spiegel, bases her piece on an interview of her grandfather and she also interviews many of the major players.
My conclusion is that there was a great deal of activism in the early 1970s, without which the studies of Dr. Hooker and Dr. Kinsey would not have been considered. There was also a generational change in progress, with younger psychiatrists who had other concerns than the status of homosexuality challenging the older conservatism. Science traditionally changes slowly and painfully, as seen from Galileo Galilei to Charles Darwin to Hooker and Kinsey. That people are upset by the changes does not make the science wrong. In this case, the APA was correct to change.
Let me conclude by personalizing this. I have never been incarcerated and am honorably discharged from the U.S. Navy. My relationship with my parents is fairly normal. I was neither excessively beaten as a child nor was I overly coddled. I have never been raped and never been molested (other than by the TSA, as is the case for many when flying these days). Most people passing me on the street would probably guess that I am straight as I don't tend to lisp or swish or any of the other stereotypical behavior associated with homosexuals. I was born under Eisenhower, thus I have well exceeded the life expectation of disproven studies like that of Paul Cameron. Neither my partner nor I have any sexually transmitted diseases. We have been together since 2005.
In other words, the reasons to consider homosexuality a pathological condition are not apparent in me, in my partner, or in any of our friends. The only problems with our relationship as I type this are that we cannot be recognized legally. The problem is discrimination, not who we are.