Rishathra

In Larry Niven's Ringworld novels, rishathra is "sexual practice outside one's own species but within the intelligent hominoids." It is not generally considered a taboo and is often used by the myriad hominids of the Ringworld as a way of sealing agreements, such as trade contracts and peace treaties. Humans, though not native to the Ringworld, share a common descent with the hominids of the Ringworld and may participate freely in rishathra.