This may relate to fetishes such as voyeurism or cuckolding. Sexual relations between women have been illustrated as well as narrated, but much of the written material from the early modern period has been destroyed. Reflective of its views on sexuality and culture, Japanese pornography delves into a wide spectrum of heterosexual, homosexual, and transgender sexual acts in addition to unique fetishes and philias. Although interracial pornography theoretically can apply to depictions of sexual activity between performers of any different racial groups, the term is most commonly used for heterosexual sex acts between black and white performers. The phrase is commonly found on porn sites. This style of filming hentai sex is in contrast to having a separate, third-person camera videos porno gay crew filming all the action. Sexologist Alfred Kind suggested that the buttocks is the primary sexual presentation site in primates. Some women who engage in same-sex sexual activity may reject not only identifying as lesbians but as bisexual as well, while other women self-identification as lesbian may not align with their sexual orientation or sexual behavior; sexual identity is not necessarily the same as one sexual orientation or sexual behavior, due to various reasons, such as the fear of identifying their sexual orientation in a homophobic setting. Sadomasochism is practiced in isolation relatively rarely, though some masochists report biting, pinching, or even stun-gunning themselves as a prelude to, or as part of, masturbation. There may be more than two participants; both group sex and gang banging can be included. Women oriented pornography is sometimes referred to as sex positive pornography. Since development and pronunciation of the buttocks begins at menarche and declines with age, full buttocks are also a symbol of youth. Some do not use the diapers at all, for arousal, or naked milf bladder and bowel movements. Though there have been numerous gang bang pornographic films since the 1980s, they usually involved no more than half a dozen to a dozen men.