Interracial pornography typically employs ethnic and racial stereotypes in its depiction of performers and many of interracial pornographic films still include racial stereotypes, although the segregation of actors by race has diminished considerably. Although pornography has usually focused on heterosexuality, due to the prevalence of the heterosexual orientation, explicit gay material has a long history as well, which reaches back to Greek antiquity, if not to prehistory. These scenes may either have the cameraman directly engaging in sex or merely filming others having sex. Since at least 1973, it has also been used to refer to a sexually attractive young woman. Squirt pornography involves women ejaculating. Voyeur or peeping Tom pornography is usually more staged with hentai sex the subjects aware of the cameras, sometimes openly including this exhibitionist awareness. Twink pornography features younger performers, and usually features young men under 30 who identify as gay or bisexual. Masturbation has been depicted in art since prehistoric times and is gay bear porn mentioned and discussed in very early writings. When sexual activity is centered on one of the threesome, then the activity may be described as a gang bang of the person. One or more of the participants may engage in autoerotic sexual activity, such as masturbation, possibly without physical contact with the other participants. The sub-genres usually conform to certain conventions, and each may appeal to a particular audience. Matsuo used amateur actresses in his mature milfs videos, and he usually traveled to their hometowns for the filming. BDSM has become an umbrella term for certain kinds of erotic behavior between consenting adults. Clothed female, naked male is a genre of erotica featuring one or more nude men and one or more clothed women. For uncircumcised males, stimulation of the penis in this way comes from the pumping of the foreskin, in which the foreskin is held and slid up and down over the glans, which depending on foreskin length, is completely or partially covered and then uncovered in a rapid motion.