As a form of non-penetrative sex, it can be done for its own enjoyment or as foreplay. Diaper fetishism, nappy fetishism or diaperism, is a sexual fetish in which a person feels a desire to wear or use diapers. This makes varying degrees of stimulation an integral part of oral sex for hentai movies many men. The industry generally refers to such films as adult films, which is part of adult entertainment. The glans itself may widen and lengthen as the stimulation continues, becoming slightly darker in colour, while the gliding action of the foreskin reduces friction. Some people get sexual pleasure by inserting objects, such as urethral sounds, into the urethra , a practice known as urethral play or sounding. Preggo pornography features pregnant performers and may include erotic lactation as a fetish. Felching is a sexual practice involving the act of orally sucking semen out of the vagina or anus of one partner. The queening stool is also related to a smotherbox, which also allows the person under the seat to be locked in place, restrained by the neck as in a set of stocks. Additionally, the buttocks give an indication of the shape and size of the pelvis, which impacts reproductive capability. Following momporn the introduction of the Motion Picture Association of America film rating system, independent animation producers attempted to establish an alternative to mainstream animation. Commenting on why there are not as many female macrophiles, psychologist Helen Friedman theorized that because women in most societies already view men as dominant and powerful, there no need for them to fantasize about it. Patricia Hill Collins, an African-American writer, theorizes that this is derived from a stereotype that those of black descent are promiscuous, and their buttocks are objectified by porn as a result. Women with women Main article: Lesbianism in erotica: Lesbianism in contemporary pornography gay twinks Lesbian or girl-girl pornography features women engaged in sex together, but is typically aimed at a male audience.