Q. Is it adultery for a man to have more that one wife?
A. I would have to say if someone has multiple wives as we find with the patriarchs in the Old Testament, it would not be adultery as long as they were married. This was permitted by God in the Old Covenant as a concession to ancient pagan practices. Long after Adam and Eve, God needed to wean His chosen people away from paganism.
Since God gave Adam only one wife we are convinced God desires that ideally marriage be between one man and one woman.
Also, in the New Covenant we find that anyone who desired to be a leader in the Church of Christ, bishops, priests, elders, deacons could only be the husband of one wife.
I Timothy 3:2 Now the bishop must be above reproach, the husband of but one wife, temperate, self-controlled, respectable, hospitable, able to teach,
1 Timothy 3:12 A deacon must be the husband of but one wife and must manage his children and his household well.
This would exclude men who had divorced and remarried as well as men with more than one wife from legitimate leadership roles in the Church of Jesus Christ.