To save myself time, I'm gonna map it out in standard logic form like this.
-The definition of a mental illness is any mental state that impairs one's normal cognitive, emotional, or behavioral functioning.
-Ignoring alternative lifestyle advocacy, the normal behavioral human functioning is to be heterosexual, simply because humans as a species (or any sexually reproducing lifeform) could not physically exist without it being a normal function.
-Homosexuality is a naturally occurring mental state that a human is born with.
-Despite the fact you can adopt a child or use artificial insemination, hypothetically a human's normal behavior (i.e. reproducing) cannot take place if they are homosexual, simply because they will never be able to have children.
-Therefore, because homosexuality, a mental state, interrupts the normal course of human behavior, it matches the definition of a mental state.
Now before I'm branded a homophobe or someone restates I lived in the 1950s(???), you have to realize this is a purely logical argument. I'm not stating that gays are bad people or that they should be deprived of the rights or normal people, I simply believe it's foolish to ignore sound facts simply to appease the group associated with it. Temporary Anxiety is classified as a mental illness. I occasionally suffer from bouts of anxiety. Do I like the fact you could tell me I have a mental illness and be correct? Not really, and a lot of people wouldn't just because it has a negative connotation typically. However, that doesn't mean anxiety shouldn't be classified as a mental illness anymore.