Transsexual porn star Wendy Williams has stated that transsexual porn is classified as Straight Specialty. Saliva pornography involves salivation and saliva. Pornographic films are generally classified into sub-genres which describe the sexual fantasy which the film and actors attempt to create. Although pornography has usually focused on heterosexuality, due to the prevalence of the heterosexual orientation, explicit gay material has a long history as well, which reaches back to Greek antiquity, if not to prehistory. The anus has a relatively high concentration of nerve endings and can be an erogenous zone, and the receiver may receive pleasure from external anal stimulation. Another variant of reality pornography consists of normal couples that are filmed by professionals, in which case the only obvious distinction from amateur pornography is the higher quality of production, filming and editing. Often a dominant woman, she may be called a Dom, Domme /ˈdɒm/, Femdomme, hentai porn Domina, or Dominatrix, depending on context or personal preference. As with most forms of sexual activity, anal sex participants risk contracting sexually transmitted infections . Individuals who can change between top/dominant and bottom/submissive roles—whether from relationship to relationship or within a given relationship—are known as switches, though the term switch in this context is occasionally seen as derogatory and is rejected by many who might simplistically fit the definition. Teacher and Student or Boss and Secretary pornography involves sexualized situations which take place between an authority figure, such as a teacher or boss, and an underling, gay boy porn such as a student or secretary, often fake. Tentacle rape or shokushu goukan is found in some horror or hentai titles, with tentacled creatures having sexual intercourse with female characters. Ass to mouth pornography shows the transfer of a performer naked milf penis from a second performer anus to that same second performer mouth in a mobile porn tubes single shot.