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. Bukkake is a sex act portrayed in pornographic films, gay sex video in which several men ejaculate on a woman, or another man. Rainbow parties were covered on The Oprah Winfrey Show in 2003, and became the subject of a juvenile novel called Rainbow Party. Hardcore pornography is pornography that depicts porn milf penetration or lesbian hentai extreme fetish acts, or both. 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. Reality and voyeur pornography, animated videos, and legally prohibited acts also influence the classification of pornography. Participants who exert sexual control over their partners are known as dominants or tops while participants who take the passive, receiving, or obedient role are known as submissives or bottoms. People may experience pleasure from anal sex by stimulation of the anal nerve endings, and orgasm may be achieved through anal penetration – by indirect stimulation of the prostate in men, indirect clitoral/G-spot stimulation in women, and associated sensory nerves . Threesome can also refer to a love triangle, a three-way romantic relationship. Sigmund Freud considered foot binding as a form of fetishism. Some aspects of the research community distinguish between female ejaculation and what is colloquially known as squirting or gushing. Some men also enjoy anal stimulation, with fingers or otherwise, without any prostate stimulation. Internationally, hentai is a catch-all term to describe a genre of anime and manga pornography. Solo performances in gay pornography have been described in 1985 as either or both active and/or passive , whereas female solo performances are said to be exclusively passive . Transsexual pornography is a genre of pornography. Once insertion is complete, the fingers are either clenched into a fist or kept straight.