As with most forms of sexual activity, anal sex participants risk contracting sexually transmitted infections . A sexual fetish may be regarded as an enhancing element to a romantic/sexual relationship achieved in ordinary ways or as a mental disorder/disorder of sexual preference if it causes significant psychosocial distress for the person or has detrimental effects on important areas of their life. It spawned numerous sequels, several of which won adult film awards. However, the tradition of bondage magazines continues in the form of art books of bondage photographs, published by mainstream publishers such as Taschen. Male masturbation may differ between males who have been circumcised and those who have not. Group sex is sexual behavior involving more than two participants. Hentai is the various forms of Japanese pornography, including H-manga, H-anime, and H-computer games. 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. Mammary intercourse is said by Alex Comfort to produce mutual orgasm in women with sensitive breasts , and it was one of the nine substitute exercises for penetrative sexual activities, as detailed in the Paradis Charnels of 1903. Can also include momporn auto-fellatio, or performing oral sex on oneself. Participants usually derive pleasure from this, even though many of the practices—such as inflicting pain or humiliation or being restrained—would be unpleasant under other circumstances. Bisexual DVDs sell much better online than in adult video stores, hentai sex possibly due to customers in stores feeling embarrassed to buy them. This may relate to fetishes such as voyeurism or cuckolding. In many cases, that person is also aroused by smelling body parts that have a urine scent. Targets the male audience, but gaysex is much less popular than Yaoi. The exact source and nature of the fluid continue to be a topic of debate among medical professionals, which is also related to doubts over the existence of the G-Spot.