None has ever suggested that Isaac practiced polygamy in any form. An examination of his life reveals that he was actually an example of faith in God, believing that a son would be provided for him just as had happened with his own miraculous birth to Abraham and Sarah.
Here is what happened. Like her mother-in-law Sarah, Rebekah his wife could not bear children. Isaac did not use this as an excuse to find a handmaiden or some other potential “surrogate mother” to bear him a son, as happens in so many different contexts today