Polygamy is Biblical.
show me in the NT anywhere it says that, where it discusses what marriage is.
Not really. From the start, it was not God’s plan, sin and man’s lustful pride to dominate over women is what led to it, and while it did exist, wherever it did exist had terrible consequences within the Family and more importantly, among the People of Israel [Think of Samuel]. So while Polygamy existed during the periods accounted for during the OT, again it was not part of God’s plan as Christ himself revealed, Christ the Groom, and one Bride, the Church.