In Japanese, the term describes any type of perverse or bizarre sexual desire or act; it does not represent a genre of work. Lesbian pornography typically features a wider range of appearances and body types than is typical of mainstream pornography. Works in this genre may explore philosophical, political or social issues. This technique is also used by some men who are circumcised who have excess skin left from circumcision. Some people masturbate by using machines that simulate intercourse. Following the introduction of the Motion Picture Association of America film rating system, independent animation producers attempted to establish an alternative to mainstream animation. The cover is open while the submissive lies down on his back and places his head in the box. HIV is among the infections that require such direct contact and is therefore very unlikely to be transmitted via mammary intercourse. Point of view pornography is adult entertainment filmed to look as if the watcher were experiencing the sex act themselves. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. To avoid soreness from this resulting friction, some may prefer to use a personal lubricant, masturbation cream or saliva during masturbation. Many of these activities might be based on rape. Erotic spankings are commonly combined with other forms of sexual foreplay. Rubber pornography involves rubber, latex, and similar materials. Mutual masturbation foreplay The manual stimulation of each other genitals where the session eventually leads to sexual intercourse. As the prostate is touch-sensitive, some directly stimulate it using a well-lubricated finger or dildo inserted through the anus into the rectum. Before the enforcement of the Hays Code, some cartoon shorts contained humor that was aimed at adult audience members rather than children. Creampie pornography features ejaculation inside the vagina , mouth or rectum instead of the common cum shot, followed by the visible seeping celebrity sex or dripping of semen from the vagina, mouth or anus.