Since mammary intercourse is a non-penetrative sex act, the risk of passing a sexually transmitted infection that requires direct contact between the mucous membranes and pre-ejaculate or semen is greatly reduced. The industry generally refers to such films as adult films, which is part of adult entertainment. This can, however, redirect semen into the bladder . It is geared towards heterosexual men, who most typically define a threesome as such. Most bisexual porn is made by small production companies rather than the major studios. They may regard it as an enhancing element of their sex lives or as their primary form of sexual activity. Bukkake is a sex act portrayed in pornographic films, in which several men ejaculate on a woman, or another man. Sub-genres can also be classified into the characteristics of the performers or the type of sexual activity on which it concentrates, and not necessarily on the market to which each sub-genre appeals. Foot fetish pornography features heavy emphasis on the feet of the participants, either in combination with momporn kissing and genital sex or in the context of acts such as trampling or crush fetishism. Because the viewing of such films carried a social stigma, they were viewed at brothels, adult movie theaters, stag parties, at home, in private clubs and also at night gaysex cinemas. Pro-am pornography is sub-genre of amateur pornography that refers to professionally made porn that features amateur actors/actresses, often appearing for the first time in a pornographic film. Biological anthropologist Helen B. Use of the agreed safeword is seen by some as an explicit withdrawal of consent. Bear features a combination of middle-aged, hairy or overweight males. Double Stuffed , Double Vaginal , or Double Anal pornography involves the penetration of one orifice by two penises or other objects. The other person may do the same during or after. Tentacle erotica can be consensual, but frequently contains elements of non-consensual sex. Women who perform in lesbian pornography hentai videos often do not consider themselves lesbian or bisexual.