A fundamental purpose of marriage is to ensure, through contractual arrangement, entitled by the legal and social institution to ensure a stable environment for the offsprings to grow up the best way possible.
On the other hand, one man for one woman, this is to ensure, everybody gets a spouse and, at a minimum, has an equalright to sex. Human sex life is in a way, moderated, by this form of social re-engineering, moving a bit away from the rules defined by natural law of attraction.
Only in this way, overall human survival and probability of having offsprings are maximized. And voila, we all survive and thrive and children grow well.