The notion of polygamy sprang from the need to provide for widows. In history men were expected to die more often then women due to risky careers such as mining, soldiering and such. 51% of all children born are male. When a man died he often left a wife and kids behind who seldom had a good chance at providing for themselves. Therefore a male relative of the deceased would wed the widow. So, originally polygamy was not seen as abusive in anyway. If anything it was regarded as somewhat heroic to provide for more people.
Now, of course, men being an evil lot they soon begun to take advantage of this practise and I can see why it is shunned upon in for example christian society. Nowadays though a lot of different family-constellations are regarded as, if not perfectly normal, at least normal enough. If the people involved concedes I don't see why it should be regarded as abusive.
In the case mentioned above; was it legal to marry thirteen year-olds in Utah 17 years ago?