What BS. So because only evolutionary purpose of sex is reproduction, a person who cannot have children is abnormal and is unable to have normal psychological functions and is thus having a mental disorder?
These (below) are the serious definitions of mental disorder. Funny that you talk about "serious" and refer to layman definitions.
"In DSM-IV, each of the mental disorders is conceptualized as a clinically significant behavioral or psychological syndrome or pattern that occurs in an individual and that is associated with present distress (e.g., a painful symptom) or disability (i.e., impairment in one or more important areas of functioning) or with a significantly increased risk of suffering death, pain, disability, or an important loss of freedom."
"A mental disorder is a syndrome characterized by clinically significant disturbance in an individual's cognition, emotion regulation, or behavior that reflects a dysfunction in the psychological, biological, or developmental processes underlying mental functioning. Mental disorders are usually associated with significant distress in social, occupational, or other important activities. An expectable or culturally approved response to a common stressor or loss, such as the death of a loved one, is not a mental disorder. Socially deviant behavior (e.g., political, religious, or sexual) and conflicts that are primarily between the individual and society are not mental disorders unless the deviance or conflict results from a dysfunction in the individual, as described above."
So no, homosexuality is not a disorder. It is observed less frequently than heterosexuality and that doesn't mean anything per se.
There is no "norm" defined in evolution. Evolution is in fact not meant to define norms. Evolution is merely studying the frequency of different traits at different points of time, and homosexuality is a trait that has remained in nature despite its lower frequency. Now there are two different arguments conjecturing why: one is homosexuality has transferred through homosexuals' direct/indirect role in upbringing of their relatives' children and the other argues that it's because it's not a chosen behavior. So this nature or nurture debate is futile. I wish i knew how it is related to pedophilia or necrophilia, and what it has to do with gay "marriage".