Sexual as opposed to asexual reproduction among living organisms began about 1.5 billion years ago with the likely benefit of increasing natural selection of advantageous genes, including to vary the genetic profile to counter and stay ahead of infectious organisms and agents, such as viruses, and parasites. Sexual disorders include sexual dysfunctions, e.g., erectile difficulties, and paraphilias. The term paraphilia is derived from the Ancient Greek for irregular love, that is, an attraction or love (“philia”) of the irregular (“para”). It implies being outside the normal range in the sphere of sex. This is an emotive area; before its discussion, it is standard to issue the formal disclaimer that any particular sexual behavior done in private and not causing harm to self or others does not necessarily constitute a psychiatric disorder and a need for psychiatric treatment. (In the words of early-20th-century English actress Patricia Campbell, “It doesn’t matter what you do in the bedroom as long as you don’t do it in the street and frighten the horses.”) What constitutes a paraphilia has varied throughout history and will continue to do so. For instance, sexual orientation toward members of one’s own sex is not currently considered a paraphilia, but homosexuality was a
psychiatric disorder in ICD-8. A further consideration in this area is the difference between psychological theories of sexual development (such as those of Freud, who theorized that it was not appropriate for a psychoanalyst to be either homosexual or bisexual) and political arguments (for example, about non-discrimination toward individuals not causing harm to others).