Others involve partners who naturally have a significant difference in size. In the 1970s, the hardcore pornography genre introduced the stereotypical cumshot scene as a central element of the hardcore film, in which the male actor ejaculates in a way ensuring maximum visibility of the act itself. 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. Interracial pornographic films have experienced an explosion in popularity, becoming one of the fastest-growing and biggest-selling genres. Initially, hot gay sex few animation studios in the United States attempted to produce animation for adult audiences, but later examples of animation produced for adults would 3d hentai gain mainstream attention and success. 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. Pornographic films are generally classified into sub-genres which describe the nude milfs sexual fantasy which the film and actors attempt to create. Especially popular in post-World War 2 Japanese culture. Sadomasochism does not imply enjoyment through causing or receiving pain in other situations . Failure to honor a safeword is considered serious misconduct and could even change the sexual consent situation into a crime, depending on the relevant law, since the bottom has explicitly revoked his or her consent to any actions that follow the use of the safeword . POV pornography involves camera angles designed to provide the viewer a view similar to that seen by one of the participants, whose face is never shown. An orgy is a gathering where guests freely engage in open and unrestrained sexual activity or group sex; and a bunga bunga orgy is an orgy in which participants have sex underwater, such as in a swimming pool or a hot tub. The sub-genres usually conform to certain conventions, and each may appeal to a particular audience.