Bukkake pornography involves a performer, usually female, onto whom several men or women ejaculate, usually on the face. No causal relationship is known between masturbation and any form of mental or physical disorder. Works in this genre may be considered adult for any number of reasons. Medical pornography involves sexualized medical, often fake, including gynecological pelvic examinations, naked milf rectal examinations, and pelvic or gynecological ultrasounds. Lying face down on a comfortable surface such as a mattress or pillow, the penis can be rubbed against it. By comparison, gonzo pornography puts the camera right into the action, often with one or more of the participants both filming and performing sexual acts, without the usual separation between camera and performers seen in conventional porn and cinema. For some people, non-penetrative sex is the primary sexual activity of choice above all others. Female masturbation involves the stroking or rubbing of a woman vulva, especially her clitoris, with an index or middle fingers, or both. 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. Common positions include lying on back or face down, sitting, squatting, kneeling or standing. Biological anime hentai anthropologist Helen B. While anal sex is commonly associated with male homosexuality, research shows that not all gay males engage in anal sex and that it is not uncommon in heterosexual relationships. Such documents have not been recognized as being legally binding, nor are they intended to be. Use of the agreed safeword is seen by some as an explicit withdrawal of consent. Whether or not licking and fondling is considered tea bagging was once debated on the Howard Stern Show. In Japan, point of view pornography is referred to as hamedori . There are various subcultures under this gay bear porn umbrella term and distinct difference between these subcultures. Diaper fetishism, nappy fetishism or diaperism, is a sexual fetish in which a person feels a desire to wear or use diapers.