Group sex pornography features at least two men and two women , usually more, who all perform sex acts on each other. Many cultures describe hentai movies pain as an aphrodisiac. Some may keep their hand stationary while pumping into it with pelvic thrusts in order to simulate the motions of sexual intercourse. They may regard it as an enhancing element of their sex lives or as their primary form of sexual activity. Starting with erotic stories and woodblock prints from before the 20th century, Japanese pornography evolved into distinct subcategories with the media that in addition of pornographic videos and magazines featuring live actors, there is exist categories of pornographic manga , pornographic computer games , and pornographic anime . Urolagnia is a form of salirophilia nude milfs in which sexual excitement is associated with the sight or thought of urine or urination. Among swinging couples, one of the two partners is often the driving force and the other is more passive-supportive of the addition of additional sex parties. Clothed male, naked female is one-sided female nudity in which one or more females are nude while one or more men are clothed. Although there may be no connection between coprophilia and sadomasochism , the limited data on the former comes from studies of the latter. As a form of non-penetrative sex, it can be done for its own enjoyment or as foreplay. Mammary intercourse is a sex act, performed as foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts and vice versa. Since mammary intercourse is a non-penetrative sex act, the risk of passing a sexually transmitted infection that requires direct contact gay boy porn between the mucous membranes and pre-ejaculate or semen is greatly reduced. The industry generally refers to such films as adult films, which is part of adult entertainment. Diaper fetishism, nappy fetishism or diaperism, is a sexual fetish in which a person feels a desire to wear or use diapers. It is geared towards heterosexual men, who most typically define a threesome as such.