Brother and sister shoudn't marry because of inbreeding. But you already knew that. And since you're stirring the pot, I can think of a valid reason why a man shouldn't marry more than one person. Can you?

You tell us why there shouldn't be polygomy. It was okay in the bible for thousands of years. It's existed in virtually every culture at one time or another. So why not?
_________________________
Tent makers for Christie, 2016.