Both sexes sometimes apply lubricating substances hentai movies to intensify sensation. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. 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. No causal relationship is known between masturbation and any form of mental or physical disorder. The niche popularity gay bear porn grew significantly in the latter half of the first decade of the 2000s. A spanking may be carried out with the use of a bare or gloved hand, or with any of a variety of implements, including a paddle, strap, hairbrush, feather duster or belt. In general, mature milfs as compared to conventional relationships, BDSM participants go to great lengths to negotiate the important aspects of their relationships in advance, and to take great care in learning about and following safe practices. Mammary intercourse is known as titty-fucking, titfuck, or a titjob in the United States, as well as tit wank or French fuck in the United Kingdom - the latter term dating back to the 1930s; while a more jocular equivalent is a trip down mammary lane. The most common male masturbation technique is simply to hold the penis with a loose fist and then to move the hand up and down the shaft. Reality pornography is a genre of pornography where staged scenes, usually shot in cinéma vérité fashion, set up and precede sexual encounters. Lying face down one may use the hands, one may straddle a pillow, the corner or edge of the bed, a partner leg or some scrunched-up clothing and hump the vulva and clitoris against it. Some people get sexual pleasure by inserting objects, such as urethral sounds, into the urethra , a practice known as urethral play or sounding. Examples of mentally oriented practices are education games, during which the dominant requires certain forms of behavior from the submissive. In the 18th and 19th centuries, some European theologians and physicians described it as heinous, deplorable, and hideous, but during the 20th century, these taboos generally declined.