It may occur more frequently in private situations, such as spontaneous sexual activity among three friends, or arranged in a community of like-minded swingers or planned as a once-only experience; and rarely in anonymous settings, such as at orgies or other sex parties. If one person is penetrated by two objects, it is generically called double penetration . Gonzo pornography is a style of pornographic film that attempts to place the viewer directly into the scene. Lolita pornography caters to ephebophiles. 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. The cover can have hinges or be a separate part. In humans, females generally have more round and voluptuous buttocks, caused by estrogen that encourages the body to store fat in the buttocks, hips, and thighs. Ass to mouth is a slang term primarily used to describe the withdrawal of a penis from the receptive partner anus followed by the immediate insertion into the receptive partner mouth. The term BDSM dates back to 1969; however, the origin of the term BDSM is unclear and is believed to have been formed either from joining the term B&D with S&M , or as a compound initialism from B&D, D&S , and S&M. Real school shots or videos which are sexualized may violate privacy laws in some countries. Rubber pornography involves rubber, latex, and similar materials. New York photographer Irving Klaw also published illustrated adventure/bondage serials by fetish artists Eric Stanton, Gene Bilbrew, Adolfo Ruiz and others. Many transgender people regard the term shemale as offensive, arguing that it mocks or shows a lack of respect towards the gender identity and gender expression of transgender individuals; in this view, the term emphasizes the biological porno gay sex of a person and neglects their gender. No causal relationship is known between masturbation and any form of mental or physical disorder.