Well, lets see you posted Bible verses and said it may only be wrong for one person not another.
The implication is that polygamy is ok with God.
Since you claim knowledge - cite where in the story Sarah's maid is referred to as his wife?
Typical (and actually often LDS) response to polygamy. God never commanded it, nor did he approve. All of the instances of polygamy turned out badly (usually following after false gods) and all were man’s idea, not God’s.