And the fact it is considered not: Wrong.
The APA and the American Psychiatric Association are the organizations that "Affirmed" homosexuality as not being a mental illness using poor and contradictory methods to reach this conclusion, but they are wrong in their methodology. Examples:
"It should be noted that the American Psychiatric Association does not consider pedophilia in itself as a mental disorder either. After discussing the ways that a pedophile could disclose “intense sexual interest in children,” they write:
If individuals also complain that their sexual attractions or preferences for children are causing psychosocial difficulties, they may be diagnosed with pedophilic disorder. However, if they report an absence of feelings of guilt, shame, or anxiety about these impulses and are not functionally limited by their paraphilic impulses (according to self-report, objective assessment, or both), and their self-reported and legally recorded histories indicate that they have never acted on their impulses, then these individuals have a pedophilic sexual orientation but not pedophilic disorder. (American Psychiatric Association 2013, 698)"
"Again, sexual fantasies and “intense sexual attractions” occur in the form of thought, so a 54-year-old man who has “an intense sexual interest” in children thinks repetitively about children in order to stimulate himself to orgasm. That person's thoughts, according to the American Psychiatric Association, are not disordered."
"It is alarming that a sadistic or pedophilic fantasy could be considered not to meet the criteria for a mental disorder"
Now, the sexual content:
"The mental orderliness of a sexual behavior could be (at least in part) determined from the physical orderliness of the sexual behavior. So, in regard to men who have sex with men, the physical trauma caused by penile-anal intercourse is a physical disorder; penile-anal intercourse almost always results in a physically disordered state in the anorectal area (and possibly the penile area of the inserter as well):
The optimal state of health of the anus requires the integrity of the skin, which acts as the primary protection against invasive pathogens … Failure of the mucous complex to protect the rectum is seen in various diseases contracted through anal intercourse. The act of intercourse abrades the mucous lining and delivers pathogens directly to the crypt and columnar cells allowing for easy entry … The mechanics of anoreceptive intercourse, as compared to vaginal intercourse, almost demands denuding of the protecting cellular and mucous protection of the anus and rectum. (Whitlow et al. 2011, 295–6, emphasis added)
It seems that the information in the previous paragraph is established as a solid scientific fact; it seems that a researcher, practitioner, psychiatrist, or psychologist would have to be ignorant or negligent to deny that fact.
So, one sign or indicator of whether a sexual behavior is normal or disordered could be whether or not it physically harms one or both people. It seems to be clear that penile-anal intercourse is physically disordered and it causes physical harm as well. Since many men who have sex with men desire to perform those physically disordered actions, it seems to follow that the desire to engage in such actions is disordered. Since desires occur at the “mental” or “thought” level, it follows that such male homosexual desires are mentally disordered."
"Furthermore, the body has within it various types of fluids. Those fluids are “physical,” and they have proper physical functions (again, that is simply a reality of medicine or health—the fluids in the human body have proper functions). Saliva, plasma, interstitial fluids, and tears all have proper functions. For example, one proper function of plasma is to transport blood cells and nutrients to other parts of the body."
"Semen is a male bodily fluid, and hence (unless one arbitrarily applies one's own rules to the field of medicine) semen has a proper physical function (or multiple proper functions) as well. Semen typically has within it many cells, known as spermatozoa, and those cells have a proper location to be transported to—the cervical area of the woman. A physically ordered sexual act of a male, then, would be one in which the semen physically functions properly. Hence, another criteria for a normal or “ordered” sexual behavior is one in which the semen functions properly by delivering spermatozoa to the female's cervical area."
"One cannot conclude (with Alfred Kinsey) that a human behavior is normal simply because it is more common than previously assumed—otherwise all human behaviors, including serial killing, would have to be considered normal."
"One cannot conclude (with C.S. Ford and Frank A. Beach) that there is “nothing unnatural” about a behavior simply because it is observed in both humans and animals—otherwise cannibalism would have to be considered to be natural. Most importantly, One cannot conclude (with Evelyn Hooker, John C. Gonsiorek, the APA, the American Psychiatric Association, and others) that a mental condition is not disordered because it does not result in “maladjustment,” distress, or impairment in social functioning—otherwise, many mental disorders would have to be labeled erroneously as normal. The conclusions arrived at in the cited literature are not supported by the premises proposed to be scientific fact; the faulty works cannot be considered credible sources."
"It is always best to give others “the benefit of the doubt.” Maybe the APA and the American Psychiatric Association accidentally made catastrophic logical mistakes in the literature they cite as evidence supporting the claim that homosexuality (and other sexual deviances) is not a mental disorder; that scenario is quite possible. Still, one should not be naïve and ignore the potential for powerful organizations to perform advocacy science. There are major inconsistencies in logic as well as arbitrary applications of certain principles by those upheld as “authoritative” in identifying and diagnosing mental disorders. The present summary and analysis in this paper of the literature put forth as “rigorous” and “significant” empirical evidence uncovers major deficiencies—irrelevant, outdated, and absurd literature—and calls into question the credibility of the APA and the American Psychiatric Association's discussion and identification of sexual disorders. Indeed, suspect anecdotes and antiquated data have been used in the debates surrounding homosexuality, but the evidence shows that even the authoritative sources on mental disorders are guilty of those charges."
Homosexuality is a mental illness. Case closed.