It's important to remember that polygamy is not restricted to polygyny. Under a system of gender equality, there is no good reason to deny that we must keep evolving until an adult, regardless of gender, sexual orientation, monogamy or polyamory, race, or religion is free to share love, sex, residence, and marriage (and any of those without the others) with any and all consenting adults. Polyamory, polygamy, open relationships are not for everyone, but they are for some. The limited same-gender freedom to marry is a great and historic step, but is NOT full marriage equality, because equality "just for some" is not equality. Let's stand up for EVERY ADULT'S right to marry the person(s) they love. Get on the right side of history!
There is no valid reason why polygamy shouldn't be legal. If two or more women want to marry the same man, why shouldn't they be allowed to?
Polygamy is a personal choice. It may not be everyone's choice of arrangement, but it is a choice, and it is a choice that should be respected. Just because some people think it's unethical doesn't mean it should be illegal. There was once a time when people thought interracial marriage was unethical too.