I also see no compelling reason to ban polygamy or incestuous marriage.
Any thoughts?
Assuming that the polygamy is approved by all the people involved (meaning those who are actually married or intending to marry) every time, I guess it is ok.
Incestuous marriage... I don't have much of an opinion about it, truth be told. I wonder how often it is even wanted, but of course that is hardly a reason to oppose it. I also wish some sort of care was taken to deal with the drawbacks (say, DNA testing to predict likely genetic risks - assuming, of course, that such a marriage means to produce children biologically or is at least fertile). Some psychological supervision, at least for a time before the marriage could happen, would probably be a good idea as well.
If the couple is psychologically sound and takes steps to avoid the expression of nocive recessive genes (say, by choosing to adopt), then sure, I see not a single reason to refuse them that right. I could be convinced in some other situations as well.