This type of pornography is almost exclusively hentai, as true hermaphroditic humans are not fertile and are not usually found in nature. This genre also features anime hentai male-on-male fellatio through a small hole made in a bathroom divider. A bisexual threesome may involve a man having sex with a man and a woman, the woman having sex with a man and a woman, or all three having sex with each other. The activity is done by itself or as a part of or prelude to other activities which many times involve gay movies sex. The term BDSM dates back to 1969; however, the origin of the term BDSM is unclear and is believed to have been formed either from joining the term B&D with S&M , or as a compound initialism from B&D, D&S , and S&M. Its gain in prominence has been attributed to its depiction in the film Pecker, which was released in 1998. Buttocks are sometimes emphasized in pornography, where they are often referred to as booty. Some men enjoy horny milfs masturbating during pegging. Mammary intercourse has sometimes been considered a perversion. If one person is penetrated by two objects, it is generically called double penetration . Rimjob pornography centers around anilingus. His most famous model was Bettie Page, who became the first celebrity of bondage film and photography. Group sex pornography features at least two men and two women , usually more, who all perform sex acts on each other. Boots are seen as the perhaps most fetishistic of all footwear and boots may be the most popular fetish clothing attire. Common phrases used to refer to these individuals are chicks with dicks or the derogatory term shemale, although these are generally considered pejorative among transwomen. Reading or viewing pornography, sexual fantasy or other erotic stimuli may lead to a desire for sexual release such as by masturbation. Many transgender people regard the term shemale as offensive, arguing that it mocks or shows a lack of respect towards the gender identity and gender expression of transgender individuals; in this view, the term emphasizes the biological sex of a person and neglects their gender.