In the Diagnostic and Statistical Manual of Mental Disorders , published by the American Psychiatric Association, it is classified under 302.89 – Paraphilia NOS and has no diagnostic criteria other than a general statement about paraphilias that says the diagnosis is made if the behavior, sexual urges, or fantasies cause clinically significant distress or impairment in social, occupational, or other important areas of functioning. Mammary intercourse is known as titty-fucking, titfuck, or a titjob in the United States, as well as tit wank or French fuck in the United Kingdom - the hentai videos latter term dating back to the 1930s; while a more jocular equivalent is a trip down mammary lane. A bisexual threesome may involve a man having sex with a man and a woman, the woman having sex with a man and a woman, or all three having sex with each other. Studies have found that masturbation is frequent in humans of both sexes and all ages, although there is variation. However, gay pornography in the modern world is mostly concentrated in the making mature milfs of home videos , cable broadcast and emerging video on demand and wireless markets, as well as images and movies for viewing on the Internet. Production of such films was profitable, and a number of producers specialised in their production. This is commonly called double vaginal penetration or double stuffing. The term breast fetishism is also used to refer to cultural attention to female breasts gay sex video and the sexuality they represent, with debate continuing as to whether the modern widespread fascination with breasts among heterosexual males in western societies is a sexual fetish. Locks may be used to emphasize the submissive position or the submissive hands may be fastened above his head to the box. Patricia Hill Collins, an African-American writer, theorizes that this is derived from a stereotype that those of black descent are promiscuous, and their buttocks are objectified by porn as a result. The primary subjects of pornographic depictions are pornographic models, who pose for still photographs, and pornographic actors or porn stars, who perform in pornographic films.