So these days most Christians say marriage is between one man and one woman...but the patriarchs of the OT often had many wives.... [Spirituality & Religion]

The tribes people had cultural norms vs religious norms. In that region plural wives was common. This was not promoted as ideal in the religion but only allowed. This was also for practical resons. Jews never did this as heavily as other neighbors anyway.
There were several things allowed in the old days that were not Gods preference for us.
Jesus says that a marriage is between one man and one woman.
Gods purpose is eternal. Commands can change