Older names for a pornographic film include stag film, and blue movie. Mammary intercourse is a sex act, performed as foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts and vice versa. Sadomasochism is practiced mature milfs in isolation relatively rarely, though some masochists report biting, pinching, or even stun-gunning themselves as a prelude to, or as part of, masturbation. Both forms of pornography generally contains nudity. Patricia Hill Collins, an African-American writer, theorizes that this is derived gay movies from a stereotype that those of black descent are promiscuous, and their buttocks are objectified by porn as a result. Foot fetish pornography features heavy emphasis on the feet of the participants, either in combination with kissing and genital sex or in the context of acts such as trampling or crush fetishism. Some men also enjoy anal stimulation, with fingers or otherwise, without any prostate stimulation. There has been an increase in discussion and portrayal of masturbation in art, popular music, television, films, and literature. The most common type of erotic spanking is administered on the bare buttocks, but can also be combined with bondage, in order to heighten sexual arousal and feelings of helplessness. Bestiality is illegal in many countries. The naked appearance of actress Demi Moore in the advanced stage of pregnancy on the cover of Vanity Fair magazine in 1991 marked the beginning of a period which has since seen pregnancy presented by celebrities as a glamorous state of living, while also creating a market for photographers to produce images of pregnant mothers, and for fashion stylists to introduce pregnancy styling to their business. It is typically animated pornography, as the legal age to be featured in a pornographic film in Japan is 18. If one person is penetrated by two objects, it is generically called double penetration . The strategies and tactics that are used to smuggle one of the couple into and out of the toilet undetected are as important or hentai movies almost as important as the urination.