Patricia Hill Collins, an African-American writer, theorizes that this is derived from a stereotype that those of black descent are promiscuous, and their buttocks are objectified by porn as a result. Many fantasies include the macrophile shrinking before a normal sized woman or keeping his or her normal height while the partner grows in size. Pornography may fall into more than one genre. Bukkake is a sex act portrayed in pornographic films, in which several men ejaculate on a woman, or another man. Other objects such as ball point pens and thermometers are sometimes used, although this practice can lead to injury and/or infection. Particular areas and processes of the body that change during pregnancy may also become the focus of psychological investment, but nudity or sexual activity is not always essential, and in some cases actual pregnancy is not necessary to invoke arousal. Many cultures describe pain as an aphrodisiac. Reality and voyeur pornography, animated videos, and legally prohibited acts also influence the classification of pornography. 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. Once insertion is complete, the fingers are either clenched into a fist or kept straight. For circumcised males, on whom the glans is mostly or completely uncovered, this technique creates more direct contact between the hand and the glans. Some men also enjoy anal stimulation, with fingers or otherwise, without any prostate stimulation. If dramatic skills are gay porn not involved, a performer in a porn film may also be called a model. Specific sex acts Anal Anal pornography features anal penetration with a penis, dildo, other objects, or some combination of them. Concerted deployed sexual rejection exercised on the partner can be an aspect of Dominance and Submission as well. The industry generally refers to such films as adult films, which is part of adult entertainment.