Because the purpose of the genes is to procreate, and we evolved into a binary system a long time ago. The animal kingdom has evolved multiple binary strategies. Humans pairbond, the man gathers resources and the woman nurtures. Lions have multiple females in a pride and protects against other aggressive males. Certain insects capture the mate and consume it, since they are solitary animals the impregnation and consumption are all the male can do for the female.
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