But polygamy throws the whole marriage-alliance system out of whack. What if the families of both wives end up in conflict and both want the political alliance benefits, whose side would the multiple-husband choose?
At least when the Targaryens did it, it was within their own family. There's no mention of any Targaryen King or Royal who married two non-Targaryen women.
If the rest of the Kingdoms looked down upon incest except for the Targaryens (because they had dragons to enforce their will, and by the time they died out everyone was too used to them in power), why would they be okay with polygamy for non-Targaryens?