The purpose of this kind of agreement is primarily to encourage discussion and negotiation in advance, and then to document that understanding for the benefit of all parties. When early sexologists in the late 19th century began to categorize and describe homosexual behavior, hampered by a lack of knowledge about homosexuality or women sexuality, they distinguished lesbians as women who did not adhere to female gender roles and incorrectly designated them mentally ill—a designation which has been reversed in the global scientific community. At these events, allegedly increasingly popular among adolescents, females wearing various shades of lipstick take turns fellating males in sequence, leaving multiple colours on their penises. The word shibari came into common use in the West at some point in the 1990s to describe the bondage art Kinbaku. The genre is popular enough in Japan that it is even the subject of parody. Gang bang pornography usually depicts numerous men having sex with one woman, or one gaysex man. A celebrity sex tape is typically an amateur pornographic mature milfs video recording involving one or more celebrity famous person which has, intentionally or unintentionally, been made available publicly. It has since become popular enough with couples to be discussed during an episode of Sex and the City. Sadomasochism refers to the aspects of BDSM surrounding the exchange of physical or emotional pain. Such video tapes are often released without the consent of their subjects, and could potentially damage celebrities careers. Milking pornography features lactating women being milked or milking themselves . Special forms include gay hentai erotic roleplay like ageplay, in which a difference in age, either real or enacted, formulates the background; or petplay. Scat or brown pornography features defecation, manipulation of feces, or consumption of feces, in reality or fantasy; a well-known example is the 2 Girls 1 Cup video. Breeding is sometimes followed by felching, which involves sucking the semen from the partner anus.