Particularly in regards to polygamy. One could argue that if it is okay for two adults to marry, then it should be okay for three or 4 adults, or however many to marry.

I personally have no problem with polygamy or polyandry. In fact, it is actually being more honest doing it that way. Many men have mistresses and concubines, and many women have men on the back burner. And many gays may have more than one lover besides the main one.