A related term used in the genre is the term cougar, which implies an older woman as predator. This type of pornography is almost exclusively hentai, as true hermaphroditic humans are not fertile and are not usually found in nature. Yaoi is anime or manga that features gay males . The term is typically used by the cinematographer within the narrative framework of a pornographic film, and since the 1970s has become a leitmotif of the hardcore genre. Some anthropologists and sociobiologists believe that breast fetishism derives from the breasts similarity to buttocks, but instead provide sexual attraction from the front of the body. As a form of non-penetrative sex, it can be done for its own enjoyment lesbian hentai or as foreplay. The volume of semen that is ejaculated depends on several factors, including the male health, age, degree of sexual excitement, young gay porn and the time since his last ejaculation. Many cultures describe pain as an aphrodisiac. A scene where many women celebrity nudes have sex with one man is referred to as a reverse gang-bang. Preggo pornography features pregnant performers and may include erotic lactation as a fetish. Internal ejaculation shots are a comparatively recent development in pornography; they are not found in early pornographic films. When the interest in a particular subject is obsessive, the behavior may be described as stalking. The first explicit pegging scene is believed to have been in the 1975 pornographic milf xxx film The Opening of Misty Beethoven. This genre also features male-on-male fellatio through a small hole made in a bathroom divider. Types of anal sex can also be a part of lesbian sexual practices. Solo performances in gay pornography have been described in 1985 as either or both active and/or passive , whereas female solo performances are said celebrity porn to be exclusively passive . Masturbation is the self-sexual stimulation of the genitals for sexual arousal or other sexual pleasure, usually to the point of orgasm. A bisexual threesome may involve a man having sex with a man and a woman, the woman having sex with a man and a woman, or all three having sex with each other.