If take the concept of consenting adults seriously, we must recognize the right of an adult to marry ANY and ALL consenting adults, regardless of gender. A woman, like a man, should be free to marry two men, or two women, men and women, etc. It is unjust that it is entirely legal for a woman to have children with, love, and live with two men at the same time, but not legally marry both fathers of her children even if that is what all want. All of the paperwork issues can be resolved. There is no rational reason to deny this freedom to marry as part of full marriage equality/
Throughout history, many societies have recognized plural marriage. Often, this plural marriage takes the form of one man having more than one wife. In a large subset of these cases, the model of marriage is one that treats women as property. This type of plural marriage should not be tolerated, as it problematizes the issue of consent. However, a man can also treat one woman as property, making the issue of consent problematic in the same way. As long as all parties are adults who have given their full and informed consent, plural marriage is acceptable.
I think that as long as both adults in the marriage agree that it is okay to have a plural marriage then there is nothing wrong with that, I think that you should be free to do whatever you want in that regards and as long as your not hurting anyone it should be allowed.
While it is important to recognize and condemn abuse and underage marriages, I also believe that consenting adults should be allowed to make their own choices. I can understand the perspective that more than one legal marriage could create problems in regards to the rights of legal marriages, I agree that spiritual and religious marriages should not be regulated, nor should they be prohibited. Although this country was founded upon the tenets of freedom of religious belief and practices, most of these laws against polygamy cite religious and moral differences with the "status quo" of morality in the 19th and 20th centuries. These types of arguments should have no hold over laws in the 21st century, pluralistic, "American" culture that we live in now. We cannot make moral judgments, let alone legal judgments, based upon a difference in religious belief.
No, consenting adults do not have the right to be in a plural marriage. States only offer one marriage license to each person. This is a very important law to keep on the books, or else some people would not get divorced prior to remarrying, and wouldn't even have to tell the second spouse. It's important for legal issues and to protect others that we not allow plural marriage in a legal sense, even though how people choose to live apart from a marriage license is their business.
It is my opinion that consenting adults do not have the rights to a plural marriage. Marriage is largely a financial institution, and allowing plural marriages to be recognized by the government would mean a change in policy that would have to be voted on by representatives of each state. A legal marriage is a complicated matter, and a plural marriage would only make policy matters more complicated.
... And bigamy is still illegal in the US (i guess unless you make a reality show about it). So no, legally no one can legally marry 5 wives at once. You asked if consenting adults have the right to - they do not. I guess they can all just live together like that creepy guy on the reality show (he has one legal marriage, iirc, the others are supposedly "spiritual". Tax time must be hilarious at that house. Eyikes! If they pay taxes, which they might not.
The U.S. Constitution provides individual rights over and above those of groups or unions, it does not make special distinction for marriage and each state has traditionally determined for itself what it's definition of marriage is. If the majority of a state sees fit to grant marriage rights to some or not to grant them to others, that is the province of the State and those who wish to have such priveliges granted should practice moral suasion to convince their fellow citizens to support them.