The New York Times reported the news of the change at the time:
The American Psychiatric Association, altering a position it has held for nearly a century, decided today that homosexuality is not a mental disorder. The board of trustees of the 20,000 member organization approved a resolution that said in part, “by itself, homosexuality does not meet the criteria for being a psychiatric disorder.” Persons who are troubled by their homosexuality, the trustees said, will be classified as having a “sexual orientation disturbance” should they come to a psychiatrist for help.The actual text of the resolution passed by the APA's board of trustees on December 15, 1973 included this paragraph:
For a mental or psychiatric condition to be considered a psychiatric disorder, it must either regularly cause subjective distress, or regularly be associated with some generalized impairment in social effectiveness or functioning. With the exception of homosexuality (and perhaps some of the other sexual deviations when in mild form, such as voyeurism), all of the other mental disorders in DSM-1 fulfill either of these two criteria. (While one may argue that the personality disorders are an exception, on reflection it is clear that it is inappropriate to make a diagnosis of a personality disorder merely because of the presence of certain typical personality traits which cause no subjective distress or impairment in social functioning. Clearly homosexuality, per se, does not meet the requirements for a psychiatric disorder since, as noted above, many homosexuals are quite satisfied with their sexual orientation and demonstrate no generalized impairment in social effectiveness or functioning.As Linda Hirshman says in her book Victory: The Triumphant Gay Revolution, back then homosexuality was considered "criminal, sinful, crazy and subversive" at the time. After the APA decided homosexuals were no longer considered "crazy" in 1973 a significant step towards a future of full LGBT equality was made. Of course, now that it is 40 years later full LGBT equality still has not been achieved but we are much closer to that future thanks to the actions by the APA on December 15, 1973.
Hat/tip to Box Turtle Bulletin