The true crime is what happens to the sons of these "marriages". They are very early on viewed as potential rivals for girls, even by thier own fathers. The are sunned and out cast at an early age. I've heard of some fathers, telling thier sons they will never have a wife, 'helping' them to turn gay.
Rumors, of course. But what does happen to the 'excess' males? That is the real question.
In the case of Mormons, many of the wives Joseph Smith took were already married to other men.