Okay, so polygamy is the only rational form of marriage. If person is bored with another person, he can marry another and remain with both instead of going through divorce.
Polygamy was shown to increase birth rates because the amount of married women increases in polygamy.
Now, it would be even better if there were multiple men with multiple women all in one marriage. That then solves the problem in those cases where one man is incapable of getting women pregnant, while also reducing number of single men.