Darrell, the fact that there is progression in the opinions of the biblical authors about polygamy in the Hebrew Bible and New Testament actually argues for a more logical explanation (although to be precise, the NT does not say polygamy is a sin).

Did God change his mind about what is right and wrong, like polygamy, slavery, eating pork and wearing clothes with dual fabrics? If he did, he's a vacillating creature.

It's more logical that the views of people changed over time and and as a result, their conception of what God wanted changed accordingly. The books assign to God's will their own prejudices. The ancient Israelites killed their enemies and ascribed their blood lust to God (such as the prescription to take a "vagina or two" as spoils of war). Jesus had a different idea (and no way to defeat the Romans in battle even if he wanted to), so his theology was different.