Whilst there have been arguments for the idea that homosexuality has a genetic base or a result of malformations of human development, today's consensus is that homosexuality is a normal variant of human behaviour (e.g. Gonsiorek, 1982) [not resulting in — or a result of — psychological disturbances or maladjustment], and therefore it is not a mental disorder.
Homosexuality was considered to be a mental disorder in 1973, but it is not now [as of 1987].
There is an extensive write-up on the history of this on the AGLP Website, and when classifying behaviours as mental disorders, you currently need to refer to the the American Psychiatric Association [APA] publication, Diagnostic and Statistical Manual of Mental Disorders [DSM] and homosexuality had been officially classified as a mental disorder in first DSM [DSM-1] in 1952. However, the APA removed homosexuality from the DSM (AGLP, n.d.).
Presented with data from researchers such as Alfred Kinsey and Evelyn Hooker, the sixth printing of the DSM-II, in 1974, no longer listed homosexuality as a category of disorder.
Although some investigations published since Hooker's study have claimed to support the view of homosexuality as pathological, they have been methodologically weak. Many used only clinical or incarcerated samples, for example, from which generalizations to the population at large are not possible. Others failed to safeguard the data collection procedures from possible biases by the investigators – for example, a man's psychological functioning would be evaluated by his own psychoanalyst, who was simultaneously treating him for his homosexuality.
Some studies found differences between homosexual and heterosexual respondents, and then assumed that those differences indicated pathology in the homosexuals. For example, heterosexual and homosexual respondents might report different kinds of childhood experiences or family relationships. It would then be assumed that the patterns reported by the homosexuals indicated pathology, even though there were no differences in psychological functioning between the two groups.
In a review of published studies comparing homosexual and heterosexual samples on psychological tests, Gonsiorek (1982) found that, although some differences have been observed in test results between homosexuals and heterosexuals, both groups consistently score within the normal range. Gonsiorek concluded that "Homosexuality in and of itself is unrelated to psychological disturbance or maladjustment. Homosexuals as a group are not more psychologically disturbed on account of their homosexuality" (Gonsiorek, 1982, p. 74)
(Source: University of California, Davis)
After a vote by the APA trustees in 1973, and confirmed by the wider APA membership in 1974, the diagnosis was replaced with the category of "sexual orientation disturbance" and a related but less public controversy involved what became the DSM-III category of Ego-dystonic Homosexuality [EDH] (Spitzer, 1981).
During the revision process of DSM-III in the mid 1980's EDH also engendered enormous controversy. In these debates openly gay and lesbian members of the APA played a decisive role in bringing about change (Krajeski 1996). Those on the APA Advisory Committee working on the revision who wanted to retain the EDH category argued that they believed the diagnosis was clinically useful and that it was necessary for research and statistical purposes. The opponents noted that making a patient's subjective experience of their own homosexuality the determining factor of their illness was not consistent with the new evidence-based approach that psychiatry had espoused. They argued that empirical data do not support the diagnosis and that it is inappropriate to label culturally induced homophobia as a mental disorder. The APA Committee agreed with the opponents and the diagnosis of ego-dystonic homosexuality was removed from DSM-III-R (1987).
AGLP, n.d. The History of Psychiatry & Homosexuality [Online]
Available at: http://www.aglp.org/gap/1_history
Gonsiorek, J. C., 1982. Results of psychological testing on homosexual populations. In: W. Paul, J. D. Weinrich, J. C. Gonsiorek, & M. E. Hotvedt (Eds.), Homosexuality: Social, psychological, and biological issues (pp. 71–80). Beverly Hills, CA: Sage
Krajeski, J., 1996. Homosexuality and the mental health professions. In: Textbook of Homosexuality and Mental Health, ed. R. Cabaj & T. Stein, pp 17-31. Washington: American Psychiatric Press.
Spitzer, R.L., 1981. The diagnostic status of homosexuality in DSM-III: a reformulation of the issues. American Journal of Psychiatry. 138(2): pp 210–215.
PMID 7457641. DOI: 10.1176/ajp.138.2.210.