many and varied are the forms of "marriage" throughout the world.
Polyandry
"is generally found in areas where difficult physical environments or high populations impose extreme pressures on agricultural systems. It works to limit population growth and to ensure the coherence of agricultural estates. Some theorists suggest that this institutions more often occurs in societies in which women hold relatively high social status"
and Polygny   
"Demographic theory suggests that polygyny may occur because of a surplus of women that results from a high incidence of male warfare. However, polygyny occurs in many situations of relatively balanced gender ratios or even, as in the case of the Yanomamo, where males outnumber females. Accordingly, some men accumulate two or more wives only at the expense of others who never marry, or, much more usually, marry at a later age than women do."

I don't think there are any "rules" for mankind - just whatever "suits" one society at one time