Am Abend , one of the earliest pornographic films that have been collected at the Kinsey Institute for Research in Sex, Gender, and Reproduction, starts with a female masturbation scene. Scenes of multiple penetration hot gay sex are common in pornography. Caro rejected that as being a necessary conclusion, stating that female fatty deposits on the hips improve individual fitness of the female, regardless of sexual selection. However, the tradition of bondage magazines continues in the form of art books of bondage photographs, published by mainstream publishers such as Taschen. However the genre remains a very small hentai sex proportion of the pornographic DVD market; for example at porn retailer HotMovies. The Japanese sex industry uses the term Paizuri . Some couples use pornography at times for variety and to create a sexual interest or as part of foreplay. It explores the more mental aspect of BDSM. For other diaper fetishists, simply wearing a diaper, the bulking feeling and crinkling sounds associated with it, is enough to cause the fetishist a form of erotic pleasure or sexual arousal. Clothed female, naked male is a genre of erotica featuring one or more nude men and one or more clothed women. Within a BDSM or D/s context, facesitting can be an especially intense form of erotic humiliation. Pornographic films are generally classified into sub-genres which describe the sexual fantasy which the film and actors attempt to create. The anus has a relatively high concentration of nerve endings and can be an erogenous zone, and the receiver may receive pleasure from external anal stimulation. Bukkake, Gokkun, Omorashi, and tentacle erotica are few uniquely Japanese genres known lesbian milfs to western viewers. It is also often mixed with other fetishes in practice, such as voyeurism, denial, etc. One or more of the participants may engage in autoerotic sexual activity, such as masturbation, possibly without physical contact with the other participants. This category also includes pornographic home movies.