Where is homosexuality considered a mental illness?

No because technically phobias aren't classed as mental illnesses, but people can be treated for them if they are causing impairment.

Also Homophobia relates more to the hatred of homosexuals and many people ignore that it is also the fear of (as the two can be expressed through aggression). Also such phobias as Arachnophobia and Acrophobia only relate to the fear itself; the fear of Spiders and fear of flying where homophobia is used more as a term for implying the discrimination of homosexuals ("they are "homophobic") rather than the fear of homosexuals.