A Mormon recently challenged me: Where in the Bible does it say that polygamy is wrong?

Be careful of falling into the trap of thinking that every point of faith and morals has to be explicitly attested in Scripture. That isn’t the case.

Scripture indicates that for a time God did tolerate polygamy during the Old Testament. However, it was portrayed even then as a negative thing. When Scripture describes the domestic life of polygamists, it brings out consistently the negative effects of polygamy—jealousy, taunting, conflict, favoritism. (Take for example the strife between the wives of Abraham, Jacob, or Elkana; see Gn 21, 29-30, 1 Sm 1).

The problems were so clearly recognized that there had to be special legislation concerning polygamy. Thus a husband playing favorites was not allowed to deprive the children of his first wife their inheritance rights in favor of the children of a more recent wife (Dt 21:15-16). Kings were forbidden to multiply wives to themselves (Dt 17:17).

As time progressed, the problems with polygamy became more and more obvious, and it stopped being practiced.

Jesus indicated that marriage was to be restored to the state God had intended in Genesis 2. Thus Jesus prohibits divorce (Mk 10:2-9) on the grounds that it was not provided for in God’s original plan. God made one man and one woman to be together. Polygamy is ruled out similarly.