The actual definition is quite simple if you go back to the origin of the debate. In the 19th century the function of sex was simply documented to be reproduction. Hence, any sexual act that cannot, in theory, result in reproduction was thought to be a "PERVERSION OF THE SEX INSTINCT". This is what voyeurism, exhibitionism, zoophilia, homosexuality, frotteurism, necrophilia and sado-masochism have in common. None can result in pregnancy. The term "sexual perversion" was eventually included in the DSM (Diagnostic and statistical manual of mental disorders) in the early 1950's as a mental disorder. With the publication of the 3rd version of DSM in 1973-78, the sexual liberation movement had shed doubt on the legitimacy of some of the so-called sexual disorders, notably homosexuality. So the psychiatric association of America dropped the term "perversion" and replaced it with "paraphilia" in an attempt to make it less pejorative. However, though the terms changed perversion/paraphilia, the definition stayed the same: most of them refer to sexual acts that cannot result in pregnancies (if practiced in its exclusive form). Of course, techinically this means that oral sex and masturbation could also be regarded as perversions/paraphilias. It is this ridiculous conclusion that has undermined the legitimacy of parphilia as a mental disorder.