Polygamy - We probably will revisit our stance on polygamy in decades to come. Historically it's been used in a way that's profoundly imbalanced towards women (i.e. almost exclusively polygyny), but it's not hard to imagine a future where that's not the case.
The case against incest can be made on that base when eugenics are re-established and codified into law. As-is the only argument that holds water is "yuck" - one that same sex marriage proponents should be vaguely familiar with.
You conflation of marriage with breeding should be rethought. I suppose infertile couples should be allowed to marry since they cannot bear children?
Any biologists care to chime in on this? It cuts both ways, and is more complicated than that.
I don't understand how polygamy is imbalanced toward woman. They get a stake of the alpha's attention and him providing for them. In the current system, it's AFBB. The beta male foots the bill for alpha seed.