When sex means reproduction, certain proclivities may simply not be part of cultural models of sexuality.
Barry and Bonnie Hewlett had been studying the Aka and Ngandu people of central Africa for many years before they began to specifically study the groups' sexuality. As they reported in the journal African Study Monographs, the married couple of anthropologists from Washington State University "decided to systematically study sexual behavior after several campfire discussions with married middle-aged Aka men who mentioned in passing that they had sex three or four times during the night. At first [they] thought it was just men telling their stories, but we talked to women and they verified the men's assertions."
In turning to a dedicated study of sex practices, the Hewletts formally confirmed that the campfire stories were no mere fish tales. Married Aka and Ngandu men and women consistently reported having sex multiple times in a single night. But in the process of verifying this, the Hewletts also incidentally found that homosexuality and masturbation appeared to be foreign to both groups.
While the Aka and the Ngandu live in the same general region, an area in central Africa marked by tropic forest, their cultures are distinct. The Aka are foragers and, according to the Hewletts, "gender egalitarianism among the Aka is about as pronounced as human societies get." Women may hunt, even on their own, and often control distribution of resources. The Ngandu, by contrast, are slash-and-burn farmers with stable locations and significant gender inequality, with men typically dominating over women.