Bisexual pornography is a genre of pornography which most typically depicts one woman and two men who all perform sex acts on each other. Interracial pornography typically employs ethnic and racial stereotypes in its depiction of performers and many of interracial pornographic films still include racial stereotypes, although the segregation of actors by race has diminished considerably. Other possibilities are possible, such as a man having sex with two milf fuck women who are having sex with each other. English adopts and uses hentai as a genre of pornography by the commercial sale and marketing of explicit works under this label. Lesbianism has been a theme in erotic art since at least the time of ancient Rome, and many regard depictions of lesbianism to be erotic. A variation on this is to place the fingers and thumb on the penis as if playing a flute, and then shuttle them back and forth. A facial is a form of non-penetrative sex, though it is generally performed after some other means of sexual stimulation, such as vaginal sex, anal sex, oral sex or masturbation. 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 gay twinks until they are close to orgasm, stop for a while to reduce excitement, and then resume masturbating. The buttocks in human females thus contain more adipose tissue than in males, especially hentai porn after puberty. Using the term shemale for a transsexual woman often implies that she is working in the sex trade. Participants usually derive pleasure from this, even though many of the practices—such as inflicting pain or humiliation or being restrained—would be unpleasant under other circumstances. Japanese rope bondage is distinguished by its use of specific katas and aesthetic rules. Another variant of reality pornography consists of normal couples that are filmed by professionals, in which case the only obvious distinction from amateur pornography is the higher quality of production, filming and editing.