There is more to the argument than just genetic risks to children, or to the mental health of the individuals.
Mostly I'm concerned with the definition of the word being altered too much, too rapidly.
The
free legal dictionary says marriage is "
The legal status, condition, or relationship that results from a contract by which one man and one woman, who have the capacity to enter into such an agreement, mutually promise to live together in the relationship of Husband and Wife in law for life, or until the legal termination of the relationship."
So that would be a legal contract between one man and one woman, and it is accompanied by a huge list of legal rights and legal responsibilities.
Let's assume for argument that the "one man and one woman" clause of the marriage contract did not exist.
The marriage agreements include many areas. There are
local, national, and international legal rights and responsibilities that include privileged conversations, financial obligations, sexual elements, and so on. There are
religious rights/responsibilities that are defined by every religion. There are
social rights, customs, and notions about marriage.
For the legal rights and responsibilities, nearly every one of those can be conferred through other legal means. Marriage has just been a convenient place for governments to attach those rights.
If those governments want to have some way to designate those rights, then great. I have no problem with anybody sharing those legal rights.
I personally know a case of adult siblings (brother and sister) who never married and never moved out. After a few years their parents died. Now they share their family home and are living similar to a traditional family with one person earning an income and supporting both. In this case they could not legally wed, although many of the legal rights would probably be appropriate for them. They have taken the legal steps to bind most of their legal rights, such as probate rights, power of attorney, and so on. They cannot get his insurance to cover her, since they aren't legally bound to. I still don't know if that is appropriate. They are still single, and have a non-sexual relationship as far as I know.
But there are other definitions of marriage.
Marriage is not just a collection
legal rights within a country. The word has many non-legal rights and connotations. Religious rights and social rights, customs, and notions cannot be so easily assigned. There are international legal rights to consider. The word with a known definition is used by insurance companies, employment contracts, business contracts, loan contracts, and other areas, where changing the legal definition could introduce major problems.
Even if a law permits the joining of two siblings for all legal rights, the law cannot force religions to accept it as marriage.
Nor can the law force social groups to accept them as equal to marriage.
Nor can the law in this country force the definitions upon other nations, just as our nation does not respect plural marriages performed in other countries today.
A change to the definition would alter and potentially destroy countless contracts and agreements that depend on the existing definition.
The current word marriage is reserved for a larger scope of rights than just the small set of legal and sexual rights.
Because the definition of marriage includes other
NON-LEGAL rights and responsibilities, I strongly disagree that those legal rights should be called "marriage". Call it a civil union, or call it an adult designee, or call it whatever else you want.
Can they have the legal rights? Sure, if they want them. Should the assignment of the legal rights be called marriage? No way, the word already has a different meaning.
You also asked about polygamy.
I feel the same applies to that. If the people want to include additional people in their legal contract, let them do so.
This would be a much more complex set of legal changes. For example, would the rights and responsibilities be associative and reflexive? Given a polygamy with MFFF, if the man dies or divorces are the other women legally responsible for children of another? What about a MMFF group that splits into two traditional couples, how would the 'divorce' be handled? How many people could enter into the relationship? Since the legal rights include privileged communication, could gangs and organized crime use it to escape the legal system? Probate could be a nightmare. I'm sure that these legal issues could be worked out.
So:
Civil union or other name? Yes. Assigning legal rights is fine.
Civil unions with polygamy? Sure. Legal rights could be assigned.
Call it marriage? No. The word marriage refers to more than just legal rights.