Oral sex pornography frequently features a person performing fellatio, and less frequently cunnilingus. Reality pornography is professionally made porn which seeks to emulate the style of amateur pornography. Films with risqué content commenced to be produced soon after the invention of the motion picture in the 1880s. Sadomasochism is practiced in isolation relatively rarely, though some masochists report biting, pinching, or even stun-gunning themselves as a prelude to, or as part of, masturbation. 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. This makes varying degrees of stimulation an integral part of oral sex for many men. In the gay pornography industry, which uses amateurs as well as professional actors, the term gay-for-pay refers to actors labeled or believed to be straight but who engage in same-sex sexual activities for money. Internal ejaculation shots are a comparatively recent development in pornography; they are not found in early pornographic films. Diaper pornography involves performers wearing diapers, sometimes with an adult performer acting in the role of a caretaker. Lying face down on a comfortable momporn surface such as a mattress or pillow, the penis can be rubbed against it. Use of the agreed safeword is seen by some as an explicit withdrawal of consent. Other fetish Bareback pornography features performers who are not wearing condoms. Participants who exert sexual control over their partners are known as dominants or tops while participants who take the passive, receiving, hentai sex or obedient role are known as submissives or bottoms. When used as an alternative to penile-vaginal penetration, the goal may be to preserve virginity or to prevent pregnancy. Bear features a combination of middle-aged, hairy or overweight males. The naked appearance of actress Demi Moore in the advanced stage of pregnancy on the cover of Vanity Fair magazine in 1991 marked the beginning of a period gay boy porn which has since seen pregnancy presented by celebrities as a glamorous state of living, while also creating a market for photographers to produce images of pregnant mothers, and for fashion stylists to introduce pregnancy styling to their business.