Evolutionary psychologists suggest that rounded buttocks may have evolved to be desirable trait because they provide a visual indication of the woman youth and fertility. Sadomasochism refers to the aspects of BDSM surrounding the exchange of physical or emotional pain. Pornographic films are typically categorized as either softcore or hardcore pornography. Some aspects of the research community distinguish between female ejaculation and what is colloquially known as squirting or gushing. Bondage pornography specializes in the depiction of sexual bondage or BDSM activities as photographs, stories, movies, or drawings. Examples of mentally oriented practices are education games, during which the dominant requires certain forms of behavior from the submissive. 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. It is the most common form of sexual hentai videos fetishism for otherwise non-sexual objects or body parts. A celebrity can fight the release in court to maintain deniability while still enjoying the career benefits. Lesbian pornography typically features a wider range of appearances and body types than is typical of mainstream milf xxx pornography. Behind the Green Door was one of the first pornographic films to feature sex between a white actress with a black actor . Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. The warm sensation felt when urine trickles on the body seems to give very relaxing and pleasurable feelings to the person. The resulting consent and understanding is occasionally summarized in a written contract, which is an agreement of what can and cannot take place.