Pornography may be presented in a variety of media, including books, magazines, postcards, photographs, sculpture, drawing, painting, animation, sound recording, momporn film, video, and video games. One or more of the participants may engage in autoerotic sexual activity, such as masturbation, possibly without physical contact with the other participants. This may help them develop their orgasm, increase its pleasure, and may inspire them to masturbate on a more frequent basis. Especially hentai porn popular in post-World War 2 Japanese culture. Within a BDSM or D/s context, facesitting can be an especially intense form gay twink porn of erotic humiliation. While bondage is a very popular variation within the larger field of BDSM, it is nevertheless sometimes differentiated from the rest of this field. Sexologist Alfred Kind suggested that the buttocks is the primary sexual presentation site in primates. Twink pornography features younger performers, and usually features young men under 30 who identify as gay or bisexual. Homi Bhabha defined the idea of a racial fetish in contrast to the idea of the Freudian sexual fetish which he describes a denial of difference, where the male sees the female as a castrated male, seeing missing parts rather than a different anatomy. Comic books often featured characters being tied up and tying others up, particularly in damsel in distress plots. The nipples are erogenous zones, and vigorous stimulation of them during masturbation usually causes the penis to become erect more quickly than it would otherwise. Mutual masturbation can be practiced by males or females in pairs or groups with or without actually touching another person as indicated by the following examples of contact versus non-contact scenarios: Non-contact mutual masturbation Two people masturbating in the presence of each other but not touching. These scenes may involve the female actor calling for the shot to be directed at some specific part of her body. Crying pornography that features participating actors crying.