It's immoral and plain out wrong. Women aren't tools, you shouldn't get married to multiple women.
Yes, because it is one-sided.
assuming its not forced and everyone has been involved in the "picking" of new wives. If Marriage is a partnership, why cant you have multiple team mates? If you made it legal then you also have to let women have multiple husbands (and wives if the wish)... Given taxation would be a wreck...
It has other issues, but it is not immoral.
As long as everybody involved consents and is fine with it, I don't see any reason why it would be wrong. I wouldn't want it, but that doesn't make it immoral.
As long as all those who are participating are consenting then there is nothing wrong with polygamy.