It really won’t take much to normalize and legalize polygamy. It has a long history in the world and never disappeared from it. There really is no ethical argument against it in the U.S. any longer. If society thinks that sodomy-based “marriage” is fine, then there is absolutely no argument against polygamy—a practice that is much more normal and healthy than sodomy.
Incest might seem to be harder to legalize and normalize at first but it probably won’t take as long as some people think. A law professor has already written on it and included the following lines:
You know those opponents of marriage equality who said government approval of same-sex marriage might erode bans on polygamous and incestuous marriages? They’re right.Once America legalizes these things, even worse things will follow.