Its simple; its a personal choice if someone wants to marry 15 men or woman as long as they are all in agreement than who am I to be against it. Often the polygamy comes with the images of molestation of children and controlling relationships but that is not always the case. As long as a person is of age of marriage and a willing participant I don't see the issue
It is beneficial in many ways, and I believe we should be able to maintain our birthright of freedom of choice, as long as not harmful to others, that's what we have laws for. Since divorce exists, it makes no since not to allow group marriages. The only people that can not comprehend this is those bound by fear, selfishness, and ignorance.
Ethics as defined by Merriam Webster is; rules about behavior based on ideas of what is morally good or bad.
Having been demonstrated over and over, morals are not a certain judeo christian sects ideas but a society's accepted ideals that can and does evolve over time.
So in some circles, polygamy is ethical and in some circles polygamy is unethical. By definition it depends on the circle one views life from.
In the US, the predominant "unethical" stance is taken by certain christian sects but is not held by all sects.
Maybe the question should be, "Should Polygamy be considered ethical by society?" In which case I would also approve as it does not effects me or others any way. Of course there would be some required changes in our tax policy and in our assistance policies, but we change those all the time anyway. Other than for reasons of bigotry, I can find no reason to make or cause to make polygamy unethical.
Honestly, it's 2014- how long is it going to take some people to realise that the private lives of others are none, repeat none, of their business. Personally, I wouldn't want to be in a polygamous relationship, but if someone else did I wouldn't think any less of them or try to make them feel ashamed about it. Provided that all the people involved are aware and in agreement to a polygamous relationship, there's nothing wrong with it. The only issue I have is in cultures where men are allowed to have numerous wives but woman can only have one husband; yet again, sexism rearing its ugly head.
Polygamy destroys the security of those people who want just a single partner. Most of the people, especially christians, want to have just one partner for life. It's a basic human need to be loved fully by the one you love. That's why many couples quarrel if the other has an affair with another person. With this, polygamy will just destroy that basic need by most of the people.