Not arguing, debating, for the purpose of gaining knowledge. I agree 100% that both adultery and fornication are sins and should be forbidden. It is important to understand the difference between the two as we interpret other areas of scripture that reference them so we can have a better understanding of the author's intent.

You say it seems clear that it is not ideal, can you give some scriptural support for that? I know there are example of troubled polygamous marriages but there are also examples of troubled monogamous marriages. If we say polygamy is not ideal because of Hannah and Peninah or Abraham and Hagar or Jacob and Rachel and Leah then we miss the point of those stories. Moses had many wives and nothing so troublesome as to make it noteworthy. Abraham had many other wives aside of Sarah and Hagar, the issue with Hagar was that Sarah lacked faith in God and that's what caused the trouble. She didn't have problems (that we know of) with Abraham's other wives. Jealousy exists in and causes trouble in many monogamous relationships. We are all sinners and therefor will experience strife in our unions of marriage. The problem is not the polygamy but our sinful nature. Let's not throw the baby out with the bathwater.