Perhaps following the example set by gay pornography, they have become a popular subgenre within heterosexual pornography since the turn of the century, featuring both vaginal and anal ejaculations. The most common male masturbation technique is simply to hold the penis with a loose fist and then to move the hand up and down the shaft. Content featuring male bisexuality has been a growing trend since the advent of internet pornography. The full-weight body-pressure, moisture, sex odors and darkness can be perceived as powerful sexual attractions or compulsions. Mammary intercourse is known as titty-fucking, titfuck, or a titjob in the United gay hentai States, as well as tit wank or French fuck in the United Kingdom - the latter term dating back to the 1930s; while a more jocular equivalent is a trip down mammary lane. 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. A related term used in the genre is the term cougar, gay twink porn which implies an older woman as predator. Specific sex acts Anal Anal pornography features anal penetration with a penis, dildo, other objects, or some combination of them. The niche popularity grew significantly in the latter half of the first decade of the 2000s. Many transgender people regard the term shemale as offensive, arguing that it mocks or shows a lack of respect towards the gender identity and gender expression of transgender individuals; in this view, the term emphasizes the biological sex of a person and neglects their gender. In the gay pornography industry, which uses amateurs as well as professional actors, the term gay-for-pay refers to actors labeled or believed to be straight but who engage in same-sex sexual activities for money.