Following World War II, during a period of social repression when governments actively persecuted homosexuals, women developed networks to socialize with and educate each other. The slave is usually strictly forbidden from placing their lips directly on the body of the dominant so the practice routinely involves them receiving much of the spray all over their face, hair and body, however, another way of doing this that applies mostly to male dominants, is to place the mouth on the head of the penis and drink the urine as it is released. The consensual submission of the submissive is sometimes demonstrated to others by symbols indicating his/her belonging to the dominant, such as wearing a collar, special tattoos, piercings, a very short haircut or a bald head. Rimjob pornography centers around anilingus. As lesbian milfs women, they face concerns separate from men. Typically, fisting hentai sex does not involve forcing the clenched fist into the vagina or rectum; this is a practice called punching. Milking pornography features lactating women being milked or milking themselves . Swinging can take place in various contexts, ranging from a spontaneous sexual activity at an informal social gathering of friends to a regular social gathering in a sex club or residence. A person may be sexually penetrated multiple times simultaneously. Because some gay men consider heterosexual men to be objects of fantasy, some gay porn producers have almost certainly described some actors as heterosexual to increase sales and publicity for their product. Perhaps following the example set by gay pornography, they have become a popular subgenre within heterosexual pornography since the turn of the century, featuring both vaginal and anal ejaculations. The mouth was the most common area to be ejaculated upon. Works in this genre may explore philosophical, political or social issues. Group sex also occurs in populations of non-human animals such as bonobo apes and chimpanzees. Mammary gay twinks intercourse has sometimes been considered a perversion.