Other objects such as ball point pens and thermometers are sometimes used, although this anime hentai practice can lead to injury and/or infection. Women oriented pornography is sometimes referred to as sex positive pornography. Bondage pornography specializes in the depiction of sexual bondage or BDSM activities as photographs, stories, movies, or drawings. The term applies to the depiction of the act rather than the act itself, and so does not include live exhibitions like sex shows and striptease. The definition clash also appears with the English definition of yuri as any lesbian relationship, as opposed to its sexually explicit definition in Japan. The reverse of this, in which mature milfs nude women interact with clothed men, is called Clothed male, naked female . 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. Rainbow parties were covered on The Oprah Winfrey Show in 2003, and became the subject of a juvenile novel called Rainbow Party. It has also been recommended as a form of foreplay or safer sex. Terminology for roles varies widely within the various BDSM subcultures. It is used, for example, at fertility clinics to stimulate sperm donors. Types of anal sex can also be a part of lesbian sexual practices. Sub-genres can also be classified into the characteristics of the gaysex performers or the type of sexual activity on which it concentrates, and not necessarily on the market to which each sub-genre appeals. Examples of mentally oriented practices are education games, during which the dominant requires certain forms of behavior from the submissive. Hardcore pornography is pornography that depicts penetration or extreme fetish acts, or both. The term pearl necklace is used by some because the drops of semen deposited by the man resemble a necklace of translucent white pearls. The existence of such content in commercially available pornography is widely considered an urban legend. In human sexuality, a threesome is sexual activity that involves three people at the same time.