The term BDSM dates back to 1969; however, the origin of the term BDSM is unclear and is believed to have been formed either from joining the term B&D with S&M , or as a compound initialism from B&D, D&S , and S&M. By comparison, gonzo pornography puts the camera right into the action, often with one or more of the participants both filming and performing sexual acts, without the usual separation between camera and performers seen in conventional porn and cinema. Portrayals of lesbians in the media suggest that society at large has been simultaneously intrigued and threatened by women who challenge feminine gender roles, and fascinated and appalled with women who are romantically involved with other women. It is geared towards heterosexual men, who most typically define a threesome as such. Some aspects of the research community distinguish between female ejaculation and what is colloquially known as squirting or gushing. Mutual masturbation, mutual manual stimulation of the genitals between partners, can be a substitute for sexual penetration. Behind the Green Door was one of the first pornographic films to feature sex between a white actress with a black actor . Sexual relations between women have been illustrated as well as narrated, but much of the written material from the early modern milf fuck period has been destroyed. This genre also features male-on-male fellatio through a small hole made in a bathroom divider. Sub-genres can also be classified into the characteristics of the performers or the type of sexual activity on which it concentrates, and not necessarily gay twink porn on the market to which each sub-genre appeals. Evolutionary psychologists suggest that rounded buttocks may have evolved to be desirable anime hentai trait because they provide a visual indication of the woman youth and fertility. The term is typically used by the cinematographer within the narrative framework of a pornographic film, and since the 1970s has become a leitmotif of the hardcore genre. Following World War II, during a period of social repression when governments actively persecuted homosexuals, women developed networks to socialize with and educate each other.