Polygamy is undesirable because the woman ends up being at a disadvantage, in some way or another. Monogamy is the desirable way of life. After all, a two-wheeler is the most popular and convenient vehicle, not a three-wheeler. But those who make a two wheeler into a three wheeler live a tensed risky life. At least one of the woman lives such a life, and it is not fair to her.
Moreover, if you can't have an exclusive relationship with your life-partner then one might be ok with open marriages also, which again raises the question, why marry at all?