OK. I'll play the devil's advocate.

Legalize polygamy (polyandry or polygyny). Why not? The cons that I heard so far were based on "morality", "love", and "values". Those arguments have no legal basis.

If a man or woman can afford to have multiple spouses, and the other half (or third or quarter) are willing to be the second (or third or fourth) spouse, why do we need a law that prohibit them from doing that? Sure, he or she may make one feel "sick", but one's sickness is not a ground for a law to be written. There are benefits to having multiple spouses, in providing child care and financial stability for instance. Since it is polygamy and not only polygyny or polyandry, there is no discrimination against either gender. What does society have to loose?