The shaft skin can also be slid back and forth with just the index finger and thumb wrapped around the penis. 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. The preference could be barefoot, high-heels, flip flops, and so on, depending on the fetishist. Bra pornography involves women or men dressed in bras. The term has also applied to other professions and even companies trying to appeal to a gay demographic. Before the enforcement of the Hays Code, some cartoon shorts contained humor that was aimed at adult audience members rather than children. Some people masturbate by using machines that simulate intercourse. Andrews cross or spreader bars. Behind the Green Door was one of the first pornographic films to feature sex between a white actress gayporn with a black actor . The activity is done by itself or as a part of or prelude to other activities which many times involve sex. A smotherbox gives a more dramatic experience . In some creampie films, both gay and straight, intercourse and internal ejaculation are followed by performers licking up the semen that has dripped from their bodies. People may also find masturbating with friends helps lift the stigma they feel surrounding the act. Testosterone discourages fat storage in these areas. Such documents have not been recognized as being legally binding, nor are they intended to be. One sex worker said that mammary intercourse was one alternative used; mammary intercourse performed by a woman with large breasts felt to the client like penetrative vaginal sex. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. In general, as compared to conventional relationships, BDSM participants go to great lengths to negotiate the important aspects of their relationships in advance, and to take great care in learning about and following safe practices.