The term "homophobia" blurs the distinction between those who oppose homosexuality on principled grounds and those who are merely prejudiced. Attempts to amplify the distinction are frequently opposed, especially by partisans who claim that all opposition to homosexuality stems from prejudice, bias, hatred, or irrational fear.
The term was invented to impugn and slander opponents of homosexuality.  It is primarily used by gay activists to portray opposition to the homosexual agenda as irrational based on fear or hatred, and to smear Christians and others.
The term is intended to sound like a form of mental illness, but unlike actual phobias, it is not a term for any recognized psychological condition. The National Association for Research & Therapy on Homosexuality says,
- "The term "homophobia" is often used inaccurately to describe any person who objects to homosexual behavior on either moral, psychological or medical grounds. Technically, however, the terms actually denotes a person who has a phobia—or irrational fear—of homosexuality. Principled disagreement, therefore, cannot be labeled "homophobia." 
Homosexuality and therapy
The therapeutic community is largely divided between proponents of gay gene theory who believe a person is born gay and will always be gay, and proponents of Reparative therapy who believe with sympathy and understanding a homosexual can be relieved of afflictions if he so desires. Gay gene advocates seek to criminalize so-called "homophobia", despite many homophobes themselves being gay, whereas Restoration therapy advocates point out the cruelty of brainwashing a young person into believing -- based upon theoretical evidence -- there is no hope of ever leading a heterosexual lifestyle.
The neologism was promoted by George Weinberg, a gay activist and psychologist, who attempted to define it in clinical literature in 1972 as "the dread of being in close quarters with homosexuals."  From there the term entered the mainstream currency through pornographic publications. 
The word comes from combining the Greek prefix homo-, meaning "same", and suffix -phobia, meaning "fear of". In early usage by homosexual activists, the term described heterosexual fears that others might think they were homosexual  The term has not been wholly accepted within the psychological therapeutic community.
Homophobia as an "Irrational Fear"
The suffix "phobia"  suggests an irrational fear, but it is most often used as a attack term by gay rights groups to suggest that opposition to homosexuality is irrational and hateful. Dr. Sander J. Breiner of NARTH has observed,
- There is no personal, internal, institutional, or cultural homophobia. The terms do not exist in the recognized scientific literature...There is only one homophobia, which has been properly defined....There is no doubt that homophobia exists. There is also no doubt that there are rationalized and irrational anti-homosexual attitudes. However, it would be very valuable for society in general, and therapists in particular, to have a clear picture of homophobia separated from all the other topics that have been lumped under that rubric." 
William O'Donohue and Christine E. Caselles  have concluded based upon research within the therapeutic community a clear understanding of the term has not been adequately evaluated and it is not clear whether the term can be accurately characterized. They have concluded,
- "the construct of homophobia, as it is usually used, makes an illegitimately pejorative evaluation of certain open and debatable value positions, much like the former disease construct of homosexuality."
- Gay Psychologist Creates New Terms for Use in the Social Debate
- NARTH Position Statements
- Plummer, David, One of the Boys: Masculinity, Homophobia, and Modern Manhood, pp. 3-4: "'Homophobia was introduced into the clinical literature by George Weinberg in 1972, in Society and the Healthy Homosexual. At that time, he referred to it as 'the dread of being in close quarters with homosexuals.'"
- Gregory M. Herek, Ph.D., published Beyond 'Homophobia': Thinking About Sexual Prejudice and Stigma in the Twenty-First Century, in the April, 2004, issue of Sexuality Research & Social Policy.
- Psychologists do not recognize it as a phobia in any generally recognized publication like the DSM IV.
- Eg, this site  first defines homophobia as a dread or fear, but says that it includes those who write their Congressmen to oppose same-sex marriage.
- HOMOPHOBIA: A Scientific Non-Political Definition, Dr. Sander J. Breiner, National Association for Research and Therapy on Homosexuality, 2003.
- William O'Donohue and Christine E. Caselles, Homophobia: Conceptual, definitional, and value issues, Journal of Psychopathology and Behavioral Assessment, Volume 15, Number 3, Springer Netherlands, September, 1993. ISSN 0882-2689