I am against polygamy both morally and in our legal systems because of my Christian faith, but I will assume you weren't looking for this viewpoint and reason otherwise.

Marriage has been around for thousands of years, and certain standards are in place to meet a variety of social needs including caring for and raising the young and the need to keep sexual activity regulated within acceptable boundaries to prevent social disorder and potentially the rampant spread of diseases such as HIV/AIDS.

But despite what marriage has traditionally been reserved for, one could argue that if you are financially stable enough to raise children (if applicable) and each partner in this polygamous relationship is 100% free of sexual diseases, than this point becomes invalid as the historical needs for marriage no longer apply in our advanced age.

So does this make every polygamous marriage worth just as much as its monogamous counterpart? Surprisingly, no. According to a study of 1,000 children tracked from birth up until ages 32, there are longstanding psychological effects dramatized by each additional sexual partner. Among various other things, the study claims that "for both men and women, taking into account prior psychological disorders, the odds of developing substance dependence increased virtually linearly with the number of sex partners."

If you would like even more evidence here is another study that discovered "significantly higher levels of rape, kidnapping, murder, assault, robbery and fraud in polygynous cultures." and also points out that monogamous marriage "results in significant improvements in child welfare, including lower rates of neglect, abuse, accidental death, homicide and intra-household conflict."