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. These styles are much closer to the style of bondage scenes in mainstream big tit milf television. Mutual masturbation, mutual manual stimulation of the genitals between partners, can be a substitute for sexual penetration. A scene where many women have sex with one man is referred to as a reverse gang-bang. Female masturbation involves the stroking or rubbing of a woman vulva, lesbian hentai especially her clitoris, with an index or middle fingers, or both. No gay twinks causal relationship is known between masturbation and any form of mental or physical disorder. Additionally, the buttocks give an indication of the shape and size of the pelvis, which impacts reproductive capability. Shotacon : Similar to Lolicon, this genre involves prepubescent or under-aged boys between the ages 6–12. 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. In humans, females generally have more round and voluptuous buttocks, caused by estrogen that encourages the body to store fat in the buttocks, hips, and thighs. The term breast fetishism is also used to refer to cultural attention to female breasts 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. Women with women Main article: Lesbianism in erotica: Lesbianism in contemporary pornography Lesbian or girl-girl pornography features women engaged in sex together, but is typically aimed at a male audience.