We assume as a given that polyandry, polygamy, or, more radically, incest wrong. I think this is just a perception because we were told this is wrong from our birth.
There are many conceivable social conditions where polygamy and polyandry must be encouraged, if not outright necessary. And a detailed reading of these holy books show at least 1-2 instances of incest, if not more (I am guessing a lot more. Quran does not have it, I think, as it is relatively recently written. But there is this instance of mohammad marrying his foster son's wife?). Incest became a taboo because it is genetically disadvantageous and people with severe disabilities were born (as is acknowledged in many advanced biology classes).
I think, the decision of practising polygamy, polyandry, homosexuality, or dare I say, (hopefully nobody starts abusing) incest is not morally indefensible if it practiced by consenting individuals. Of these polygamy and polyandry is more so not the business of anybody else, if all individuals consent. Incest, on the other hand, I think, needs regulation as innocent kids with severe disabilities may be born.
