The custom of sexual hospitality recorded in Marco Polo’s The Description of the World,that is,to serve guests with wives and daughters,is a uniquely customary material.This paper shows that in lots of historical archives written in Chinese and other languages,similar records can also be identified.This indicates that the phenomenon is not an individual case in the history of human custom,and its traces can be discovered in the early stage of the development of many Eurasian ethnic groups.This also means that the formation and maintenance of such a custom is attributed to the biological demand of remote ethnic groups,that is,to acquire different genes.At the same time,the socalled to please the deity is actually the outcome conceived by those who manipulated religion to help rationalize and keep this custom.This research,actually,proves the authenticity and historical value of The Description of the World.