Mutual masturbation, mutual manual stimulation of the genitals between partners, can be a substitute for sexual penetration. Behind the Green Door was one of the first pornographic films to feature sex between a white actress with a black actor . The term BDSM dates back to 1969; however, the origin of the term BDSM is unclear and is believed to have been formed either from joining the term B&D with S&M , or as a compound initialism from B&D, D&S , and S&M. Some prefer a particular type of clothing to urinate. These are often fake. Evolutionary psychologists suggest that rounded buttocks may have evolved to be desirable trait because they provide a visual indication of the woman youth and fertility. Some anthropologists and sociobiologists believe that breast fetishism derives from the breasts similarity to buttocks, but instead provide sexual attraction from the front of the body. It has since become popular enough with couples to be discussed during nude milfs an episode of Sex and the City. Crush pornography anime hentai features objects being crushed by an actress or actor, who generally plays an overtly dominant role. The queening stool is also related to a smotherbox, which also gay movies allows the person under the seat to be locked in place, restrained by the neck as in a set of stocks. 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. A scene where many women have sex with one man is referred to as a reverse gang-bang. Erotic spankings are commonly combined with other forms of sexual foreplay. Arcane or occult pornography involves sexualized situations with an occult or magical tone or performers who costume themselves in keeping with the theme, for example, a male performer dressed as a wizard. 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.