Social attitudes towards the discussion and presentation of sexuality has become more tolerant and legal definitions of obscenity have become more limited, leading to an industry for the production and consumption of pornography in the latter half of the 20th century. Bukkake videos are a relatively prevalent niche in contemporary pornographic films. Erotic literature and pornographic films have been devoted to the sexual uses of glory holes. Seconds after being deposited onto the face, the semen thickens, before hentai porn liquefying 15–30 minutes later. It is also often mixed with other fetishes in practice, such as voyeurism, denial, etc. Facesitting is common among dominant and submissive individuals, for demonstrating superiority and for sexual gratification. There are no particular or preferred elements naked celebrities within maiesiophilia that are common to all maiesiophiliacs. The stimulation may involve hands, fingers, everyday objects, sex toys or combinations of these. Scenes of multiple penetration are common in pornography. Though the term anal sex most commonly means penile-anal penetration, sources sometimes use the term anal intercourse to refer exclusively to penile-anal penetration, and anal sex to refer to any form of anal sexual activity, especially between pairings as opposed to anal masturbation. However the genre remains a very small proportion of the pornographic DVD market; for example at porn retailer HotMovies. Fisting involves the insertion of one partner entire hand into a vagina or the anal cavity. Fistees who are more experienced may take two fists . Another aspect is the structured training of the Bottom. Female-to-male strap-on pornography became a popular momsex sub-genre in the United States in the early 21st century, with a number of studios producing dedicated series about it. Not so for another pair of Czech twins, Elijah and Milo Peters, who work together condomless for both oral and anal gayporno sex for studio Bel Ami. Some couples use a threesome as a gay bear porn w celebrity porn ay to develop a love triangle .