Sub-genres can also be classified into the characteristics of the performers or the type naked milf of sexual activity on which it concentrates, and not necessarily on the market to which each sub-genre appeals. Masturbation is the self-sexual stimulation of the genitals for sexual arousal or other sexual pleasure, usually to the point of orgasm. Behind the Green Door was one of the first pornographic films to feature sex between a white actress with a black actor . Swinging can take gay twinks place in various contexts, ranging from a spontaneous sexual activity at an informal social gathering of friends to a regular social gathering in a sex club or residence. A 2009 study found that heterosexual male fat admirers preferred females that were clinically overweight and rated both overweight and obese women more positively than slighter individuals. Positions can also be chosen specifically for added effects such as increased humiliation, elevation and suspension. Top and dominant are widely recognized for those partner in the relationship or activity who are, respectively, the physically active or controlling participants. Group sex pornography features at least two men and two women , usually more, who all perform sex acts on each other. Bondage pornography specializes in the depiction of sexual bondage or BDSM activities as photographs, stories, movies, or drawings. No causal relationship is known between masturbation and any form of mental or physical disorder. The activity is done by itself or as a part of or prelude to other activities which many times involve sex. A lesbian is a female homosexual; a female who experiences romantic love or sexual attraction to other females. Felching is a sexual practice involving the act of orally sucking semen out of the vagina or anus of one partner. The loose and direct camera work often includes tight shots of the genitalia, unlike some traditional porn. Both sexes sometimes apply lubricating substances to intensify sensation. Lesbian pornography typically features a wider range of appearances and body types than is lesbian hentai typical of mainstream pornography.