I wouldn't be so sure about that. There are many circumstances when it is actually encouraged to marry more than once, some of which have already been mentioned. But let's take the most 'trivial' one as an example : if a man isn't satisfied with one wife alone and he feels he fulfills the conditions for taking more wives, then it is better for him to marry than stay monogamous and try to control his desires. I think a lot of the time, it has to do with what his own wives deem as 'fair'.