Buttocks are sometimes emphasized in pornography, where they are often referred to as booty. Bear features a combination of middle-aged, hairy or overweight males. Perhaps following the example set by gay pornography, they have become a popular subgenre within heterosexual pornography since the turn of the century, featuring both vaginal and anal ejaculations. Bukkake is a sex act portrayed in pornographic films, in which several men ejaculate on a woman, or another man. Such ejaculation can be done by mammary intercourse, or masturbation over the other person, or result from the withdrawal of the penis from a partner mouth as he is ejaculating following fellatio, irrumatio, or intercourse. There has been an increase in discussion and portrayal of masturbation in art, popular music, television, films, and literature. Mammary intercourse is a sex act, naked milf performed as foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts and vice versa. Though there have been numerous gang bang pornographic films since the 1980s, they usually involved no more than half a dozen to a dozen men. These actors often play the top roles but this is not always the case, such as with Kristen Bjorn and some gay twinks Bel Ami models. When used as an alternative to penile-vaginal penetration, the goal may be to preserve virginity or to prevent pregnancy. This type of pornography is almost exclusively hentai, hentai porn as true hermaphroditic humans are not fertile and are not usually found in nature. Contrary to the other two types, B/D does not define the Tops and Bottoms itself, and is used to describe the general activities with either partner being the receiver and the giver. Hermaphrodite, also known as herm, refers to pornography, typically hentai, that features persons with the genitalia of both sexes. The loose and direct camera work often includes tight shots of the genitalia, unlike some traditional porn. Cuckolding can also be mixed with other non-monogamous relationship arrangements with which it has substantial overlap such as Swinging, wife-swapping, open relationships, and polyamory, etc.