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. Among swinging couples, one of the two partners is often the driving force and the other is more passive-supportive of the addition of additional sex parties. It is also known colloquially as gushing or squirting, although these are considered to be different phenomena in some research publications. Mammary intercourse is a sex act, performed as foreplay or as non-penetrative sex, that involves the stimulation of the male penis by the female breasts and vice versa. Transsexual pornography is a genre of pornography. A sex party is a gathering at which sexual activity takes place. There has been an increase in discussion and portrayal of masturbation in art, popular music, television, films, and literature. The full-weight body-pressure, moisture, sex odors and darkness can be perceived as powerful sexual attractions or compulsions. Terminology for roles varies widely within the various BDSM subcultures. Foot fetishism has been defined as a pronounced sexual interest in the feet or footwear. Since mammary intercourse is a non-penetrative sex act, the risk of passing a sexually transmitted infection that requires direct contact between the hentai porn mucous membranes and pre-ejaculate or semen is greatly reduced. 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. Diaper pornography involves performers wearing diapers, sometimes with an adult performer acting momporn in the role of a caretaker. Sigmund Freud considered videos porno gay foot binding as a form of fetishism. While anal sex is commonly associated with male homosexuality, research shows that not all gay males engage in anal sex and that it is not uncommon in heterosexual relationships. Because the viewing of such films carried a social stigma, they were viewed at brothels, adult movie theaters, stag parties, at home, in private clubs and also at night cinemas.