However, the tradition of bondage magazines continues in the form of art books of bondage photographs, published by mainstream publishers such as Taschen. 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. Group sex can occur between people of all sexual orientations and genders. Going back at least as far as the Christy twins in the 1970s, depictions of incest and particularly incest between twins has been a feature of gay pornography. For other scenes, particularly in established relationships, a safeword may be agreed to signify a warning rather than explicit withdrawal videos porno gay of consent; and a few choose not to use a safeword at all. Various groups within society have considered depictions of a sexual nature immoral, labeling them pornographic, and attempting to have them suppressed under obscenity hentai sex and other laws, with varying degrees of success. However the genre remains a very small proportion of the pornographic DVD market; for example at porn retailer HotMovies. But starting with The World Biggest Gangbang , starring Annabel Chong, the pornographic industry began producing a series of films ostensibly setting gangbang records for most consecutive sex acts by one person in a short period. Initially, few animation studios in the United States attempted to produce animation for adult audiences, but later examples of animation produced for porn milf adults would gain mainstream attention and success. Accepting that Eros is not always pretty, and may involve the forbidden or taboo - as with women pornography – forms part of the struggle with the good girl/bad girl or virgin-whore split in postmodern femininity. Ethnic pornography is a genre of pornography that features performers of specific ethnic groups, or depictions of interracial sexual activity.