Preggo pornography features pregnant performers and may include erotic lactation as a fetish. To avoid soreness from this resulting friction, some may prefer to use a personal lubricant, masturbation cream or saliva during masturbation. A study of 164 males in Finland from two SM clubs found that 18.2% had engaged in coprophilia; 3% as a sadist, 6.1% as a masochist, and 9.1% as both. 18% of heterosexuals and 17% of homosexuals in the study pool had tried coprophilia, showing no statistically significant difference between heterosexuals and homosexuals. Studies have found that masturbation is frequent in humans of both sexes and all ages, although there is variation. This makes varying degrees of stimulation an integral part of oral sex for many men. 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. Pornography may fall into more than one genre. Testosterone discourages fat storage in these areas. Other objects such as ball point pens and thermometers are sometimes used, although this practice can lead to injury and/or infection. Gay pornography is the representation gay movies of sexual activity between males. By comparison, gonzo pornography puts the camera right into the action, often with one or more of the participants both filming and performing sexual acts, without the usual separation between camera and performers seen in conventional porn and cinema. The term bondage describes the practice of Physical nude milfs restraining. If one person is penetrated by two objects, it is generically called double penetration . The actual height of the giantess within the fantasy may vary depending on the individual. Bukkake is a sex act portrayed anime hentai in pornographic films, in which several men ejaculate on a woman, or another man. Lolita pornography caters to ephebophiles. A scene where many women have sex with one man is referred to as a reverse gang-bang. 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.