Often, strong views are expressed with regard to anal sex; it is controversial in various cultures, especially with regard to religion, commonly due to prohibitions against anal sex among gay men or teachings about the procreative purpose of sexual activity. The smotherbox is placed on a stable surface. Depictions of male and female masturbation are common in pornography, including gay pornography. It is used, for example, at fertility clinics to stimulate sperm donors. Using the term shemale for a transsexual woman often implies that she is working in the sex trade. Emetophilia pornography involving vomiting and gagging. If this is done using penises and/or strap-on dildos, this is sometimes called the sandwich or BigMac. Sexologist momporn Alfred Kind suggested that the buttocks is the primary sexual presentation site in primates. Sex experts have praised various techniques that the performer can use during fellatio to increase their partner pleasure. Ass to mouth pornography shows the transfer of a performer penis from a second performer anus to that same second performer mouth in a single shot. A variant of A2M is ATOM where a performer gaysex inserts his penis directly from one recipient anus into a second recipient mouth. A pearl necklace is a slang term referring to a sexual act in which a man ejaculates semen on or near the neck, chest, or breast of another person. Especially popular in post-World War 2 Japanese culture. Guro or Gore is a genre of Japanese pornography involving blood, gore, disfiguration, violence, mutilation, urine, enemas, or feces. In some societies, the terms fox, hentai porn plum, chick, or chicken are preferred. Said that the female buttocks evolved in the context of females competing for the attention and parental commitment of powerful resource-controlling males as an honest display of fat reserves that could not be confused with another type of tissue, although T. Since at least 1973, it has also been used to refer to a sexually attractive young woman. These scenes may involve the female actor calling for the shot to be directed at some specific part of her body.