Pornography may fall into more than one genre. 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. Some aspects of the research community distinguish between female ejaculation and what is colloquially known as squirting or gushing. Use of the agreed safeword is seen by some as an explicit withdrawal of consent. Multiple men and women Bisexual pornography features one woman and two men who all perform sex acts on each other. The industry generally refers to such films as adult films, which is part of adult entertainment. Diaper fetishism, nappy fetishism hentai sex or diaperism, is a sexual fetish in which a person feels a desire to wear or use diapers. Boots are seen as the perhaps most fetishistic of all footwear and boots may be the most popular fetish clothing attire. Group sex pornography features at least two men and two women , usually more, who all perform sex acts on each other. Today, pornographic films can be sold or rented on DVD, shown through Internet and special channels and pay-per-view on cable and satellite, and in adult theaters. Both sexes sometimes apply lubricating substances to intensify sensation. The most common male masturbation videos gays technique is simply to hold the penis with a loose fist and then to move the hand up and down the shaft. 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. The name was coined from the title of the controversial book. What seems clear from the historical record is that much of the lesbian material in pornographic texts was intended for a male readership. For instance, Amateur Allure naked milf has a trademark shot where the sexually performing cameraman circles a handycam around the sexually performing model head. Its gain in prominence has been attributed to its depiction in the film Pecker, which was released in 1998.