It appears to me that divorce is legitimate when the contract of “one flesh” is abandoned, in which case both parties are free of the former two-party contract. Or if one party who is an unbeliever does not want to make a commitment and is prone to leave, 1 Cor. 7. Multiple wives do not make for a divorce, thus for King David and Solomon divorce from them was not legitimate which makes defining these things confusing.
Jesus answers the Pharisees in Matthew 19 that a man can not divorce his wife for any reason, and they who divorce for any reason not only commit adultery but make their spouse commit adultery. If one party in a Christian marriage has left the duty of marriage they in effect have left the duty to others and both are responsible for adultery. However, Moses in old time had allowed divorce due to the hardness of men’s hearts possibly to go beyond fornication to some degree. So it has been and is permissible but not for any reason other than adultery and fornication from the way I see it.