Lying face down on a comfortable surface such as a mattress or pillow, the penis can be rubbed against it. For example, in Australia it is rated Refused Classification . In general, BDSM play is usually structured such that it is possible for the consenting partner to withdraw his or her consent during a scene; for example, by using a safeword that was agreed on in advance. John Stagliano is considered to have started the gonzo pornography genre with his videos gays Buttman series of films. Sub-genres can also be classified into the characteristics of the performers or the type of sexual activity on which it concentrates, and not necessarily on the market to which each sub-genre appeals. Double penetration of the vagina, anus and mouth, can involve: Simultaneous penetration of the anus by two penises or other objects. Foot fetish pornography features heavy emphasis on the feet of the participants, either in combination with kissing and genital sex or in the context of acts such as trampling or crush fetishism. The term sadomasochism is derived from the words hentai sex sadism and masochism . In some scenes or relationships it may be impossible for consent to be withdrawn in the middle of a scene, or the bottom may have the ability to revoke consent for a relationship as a whole, but not for a particular scene. Fisher said that perhaps, the fleshy, rounded buttocks attracted males during rear-entry intercourse. The term applies to the depiction of momporn the act rather than the act itself, and so does not include live exhibitions like sex shows and striptease. Snowballing or snowdropping is the human sexual practice in which one person takes someone else semen into his or her mouth and then passes it to the mouth of the other, usually through kissing. The use of the word creampie to describe such scenes originated in U. The term pearl necklace is used by some because the drops of semen deposited by the man resemble a necklace of translucent white pearls. Both forms of pornography generally contains nudity.