I find there is no problem with polygamy, polyandry, or homosexual marraige.
If two men and one woman, or two women and one man wish to live their lives as a social unit, why is that bad? Just one for example, in our moden society, two can work while one stays home to take care of the children. There are many other reasons and possibilities. There is no crime and there is no victim. What is the problem?
Regarding homosexual couples, if any two people find comfort, companionship, joy in their relationship, why do I have any business in paying any attention what-so-ever to what goes on in the privacy of their home. If they wish to contribute to each others medical care, each others retirement funds, or any other endeavor, how am I harmed? I am not.
(I do draw a line with emotional and physical mistreatment, even when both parties appear to participate on a voluntary basis)
"Marriages" of one male and one female break up with all sorts of legal problems. Why should partnerships of any other form be more or less problematic? (Yes there are more poeple involved, but the same is true with large families, corporations, etc.)
The time has long since arrived for self righteous bible thumpers and manipulative governments to stop trying to push their assumed moralities on the private lives of citizens.
To address a few replies, the problems with children and beasts have probably existed since the beginnings of humanity are not in the same class as the OP. That side issue should be dropped from this thread.