When people act as if all of society's work around marriage rights is done with the advent of legal same-sex marriage, mention that not everyone is in a two-adult relationship, and mention your support of people who are in poly families who wish to be legally married. Increasing awareness of poly families will probably go further than anything else we can do.
  — How to be an Ally for Poly Marriage

Unfortunately, there is no way to add my second partner to my health insurance or to file taxes as a family. We won't be able to make loss of consortium claims, receive disability benefits for each other, or exercise any of the other hundreds of privileges belonging to married couples. Without poly marriage, my relationship with my second partner will always be treated by the legal system as less important than my relationship with my first partner.
 — The Need for Poly Marriage

One thing I haven't been able to find, however, is a detailed proposal for how the law would work. With gay marriage the law change really is as simple as changing "one woman and one man" to "two people" but while I see people claiming poly marriage is just expanding marriage from "two people" to "people" I'm not sure that's enough:

• You could either allow people to have multiple pairwise marriages or just a single group marriage. The former makes for very tricky property law, the latter makes it confusing when an existing group wants to add someone to their marriage or have a partial divorce. (If A, B, and C are married and C wants out, are A and B still married?)
• One spouse in a large family could be in the military, all other spouses would get the legal benefits of being a military spouse (Tricare eligibiliity, childcare, more)
• Social Security currently pays out spousal benefits, and if they're paying out to multiple spouses for the same person (because of divorce and remarriage) they don't scale down by the number of spouses. So many surviving spouses of one social security worker could collect benefits.
• This isn't a government question, but how should paid parental leave work with a group marriage? If a company currently gives employees N weeks off if they bear a child or M weeks off if they're married to someone who does, what if ten married people claim the N+9M weeks of paid leave for the same child?
• Gangs and organized crime could get large group marriages, and then invoke spousal privilege to avoid being required to testify against each other. Jail visiting rights would also be useful here.
• A large company could be a sole proprietorship with the employees all part of a large group marriage, gaining some tax advantages and possibly child labor exemptions.
• Immigration by marriage is currently one of the easier forms. There's already government work that goes into making sure that such marriages are legitimate and not for immigration purposes, but if 100 people are married with one citizen do we let them all in?
• A line marriage could avoid estate taxes by never actually ending.
• Many of the other 1,138 federal statutes that reference marriage (listed in this GAO report) are probably not written with multiple spouses in mind.

I'm not saying these can't be solved, or are even arguments against legalizing poly marriage, but I'm curious what the proposed solutions are.

(Maxium sizes would help with many of these, but where to draw the line? Probably you allow at least 5, but 10? 25? 100?)