Certainly in Aztec society it was acceptable for the tlatoani (ruler of the city-state, essentially) to have multiple wives and concubines. According to Spanish records, Moctezuma II had roughly 100 children and fifty women were currently pregnant when the Spanish arrived, although this is likely to be somewhat exaggerated. I am unsure of the reasoning but it certainly helped to ensure there would be a male heir for title of tlatoani.

Source: Struggle and survival in colonial America edited by David G. Sweet and Gary B. Nash. Published 1981 by University of California Press in Berkeley. (215)