The anus has a relatively high concentration of nerve endings and can be an erogenous zone, and the receiver may receive pleasure from external anal stimulation. Going back at least as far as the Christy twins in the 1970s, depictions of incest and particularly incest between twins has been a feature of gay pornography. Men and women may masturbate until they are close to orgasm, stop for a while to reduce excitement, naked milf and then resume masturbating. Since the 1980s, many practitioners and organizations have adopted the motto safe, sane and consensual, commonly abbreviated as SSC, which means that everything is based on safe activities, that all participants be of sufficiently sound/sane mind to consent, and that all participants do consent. Mutual masturbation is a sexual act where two or more people stimulate themselves or one another sexually, usually with the hands. Such films continued to be produced but could only be distributed by underground channels. Women in homosexual relationships responded to this designation either by hiding their personal lives or accepting the label of outcast and creating a subculture and identity that developed in Europe and the United States. A female dominant in a master/slave relationship is often termed a Mistress, not to be confused with the colloquial usage of mistress as a kept sexual partner without a similarly formalized power relationship. A variation on this is to place the fingers and thumb on the 3d hentai penis as if playing a flute, and then shuttle them back and forth. See Gay-for-pay. The scenario is a genre of erotica, which on the internet is sometimes abbreviated to CMNF, and may involve sexual and nonsexual erotic scenarios. The person sat upon may be in bondage, sexually submissive, or simply held down by the body-weight of the other person. In gay sex video addition to mainstream pornography, the popularity of facials has led to creation of its own niche market, like video series that specialize in showing the act. Sometimes special furniture is used, such as a queening stool or smotherbox.