Recently on the WELS discussion page it was stated that polygamy is not a sin. Martin Luther was even referenced by one person. Is it true that polygamy is not a sin?

Polygamy is a sinful deviation from God’s design that marriage be between one man and one woman (Genesis 2:20-24; Matthew 19:4-6; Romans 7:2-3; 1 Corinthians 7:2; 1 Timothy 3:2; Titus 1:6). The fact that God did not intervene in people’s lives when they went beyond his design and will does not equate to approval on his part. The biblical narratives of those people who went beyond “one man, one woman” paint a home life marred by strife and troubles. Those narratives illustrate how God’s ways are always best—including what he says about his institution of marriage.