If one man married 100 wives, 1 company insurance policy would have to have 98 more people covered on the family plan, and would in turn raise the cost of keeping the worker to astronomical levels. Also, imagine the IRS's horror when the man declares 100 dependents.
If there were no legal benefits to marriage, bigamy and polygamy would not matter so much. However, marriage brings legal benefits, some of which cost third parties money, ergo it is of interest.
In the US, 1 on 1, even for mormons. Other marriages do happen, but are null and void after the first marriage.
I would be interested in knowing how some Muslim countries deal with this. One way is to have only religious marriage, with no "legal" (think "governmental") involvement in the marriage and therefore no legal benefits.
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