Polygamy? Do you guys have any Biblical justifications or did Smith/Young have their own visions to justifiy it?
Both -if you read the Old Testament you will know that the ancient Hebrew patriarchs from Abraham to Jacob were polygamists (or at least bigamists).
True, but why did Smith/Young accept it after monogamy became the norm?
That's a good question. According to this PBS documentary (which I strongly encourage you all to watch if you haven't already), Joseph Smith supposedly received a vision during the Nauvoo, IL period that told him that polygamy was divinely sanctioned. He later pressured his lieutenant Brigham Young into (reluctantly) accepting it as well.
Well, reluctantly at first, because he ended up having 55 wives, and having 56 children from 16 of them.