I will only example some practical issues that will follow institutionalized polygamous marriage. I could engage in a higher level of generality but I do not think that is necessary to show why polygamous marriages would change the institution of marriage in the U.S. while a marriage between two persons of the same sex does not.
You are partially right about this, and I think you are going to see that the poly community is going to be okay with a civil union solution whereas the gay community was not and for good reason. They gay community was fighting for equality, and we have already accepted that you can't be equal but separate. The poly community is not fighting for equality, but to remove an abridgement of our freedom to marry.
I'll try to answer some of these, but I can't help notice that a number of these are not any different than problems already faced by a monogamous marriage.
Who will offer health insurance to employees' spouses, if an employee may have an unlimited number of spouses?
This is a problem, no doubt. I think the right argument here is that we should not be abridging my freedom in order to make it easier on the insurance companies.
How well do you think that argument would go over if they were trying to tell people they could not have more than one kid? Who will offer health insurance to employees' children, if an employee may have an unlimited number of children?
How long the spousal immunity should last when the entire gang members enter into a group marriage?
What about the 5th Amendment? How are we supposed to get gang members to testify when almost anything they testify to will incriminate them as well?
The reality is it is nearly impossible to compel a person to testify anyway. I imaging that they will do it the same way they do it now, by offering them deals that make it in their best interest to cooperate. I doubt this will make a big difference. Why don't criminals pair off and marry each other already? Because image is still important, and they won't want to be married to each other because the first group to do it will get mercilessly mocked by everyone.
How will INS monitor the naturalization processes of unlimited foreign spouses who group-marry a U.S. citizen?
How do they do it now with monogamous marriages? I imagine they will do it the same way, it just might take longer due to the extra man hours involved in investigating. This is something the poly community will just have to accept.
I personally don't think this is a problem, who even cares if a few people are able to become American Citizens by marriage?
If spouse A divorces her/his spouses out of a group marriage and remarry one of them in the future, what are the legal statuses of the other spouses in relation to the spouse A?
I have talked about this already, most of these sorts of problems are solved with a 'one marriage,many people' approach to group marriage. The answer in this case is that if 4 people are married and one wants a divorce they all get divorced, and then the 3 that still wants to be married have to get remarried. If the 4th wants back in at a later date then the 3 has to end their marriage and create a new one with the 4th.
This might sound complicated, but it is no more so than monogamous marriages. I have a monogamous friend that at age 43 has been married and divorced 5 times but has only had two wives!
Which spouse in the group marriage should have a custody of a child if the child's natural parents die in an unfortunate accident and the rest of the spouses want to divorce?
This is probably the most problematic issue there is in poly. What the poly community would like the law to be is that at the time of a child's birth everyone in the poly marriage is considered by law the child's legal parents, and that if the poly-family breaks up that all parties retain parental rights, that would include stuff like visitation and child support. If the biological parents should die, the child would remain with the rest of the poly-family.
If that poly-family should break up then a court will have to decide just who gets custody just like they do in the case of a monogamous divorce.
If a spouse in a group marriage takes a child to a foreign country and does not return, what happens?
The exact same thing as if that should happen to a monogamous couple. There is no difference here at all.
We can make these up (and multiply them) all day.
And I can answer them all day, because the answer to most of them are that it is not really different than the solutions we already use in monogamous relationships.