Ancient Israel differed from all other cultures in many ways, but one of them was their law concerning marriage. There are no Old Testament passages sanctioning or prescribing divorce. Marriage was for life. Nevertheless, the assumption was that people would get divorces and God spoke to the matter.
First, divorce should be a significant matter. A man could not just dismiss his wife. He had to go through a legal procedure to give her a certificate of divorce. Second, should she remarry, she would never be able to go back to nor be taken back by her previous husband. This ensured the second marriage.
“Sharing” a woman was akin to prostitution and just like prostitution defiled the Holy land in which God and His people lived. The law served to protect the institution of marriage, which was intimately bound up in the holiness of the nation.
In the Bible, marriage is not a human-made institution. It cannot be defined as we see fit. On the other hand, the world can define it any way they like. It won’t make such definitions acceptable to God. Christian people, God’s people, must remember that our lives are defined by God and live accordingly. The fact that God’s people have so ignored and degraded marriage with our own behavior destroys our moral credibility with the world. We have no right to demand of them what is expected of us when we ourselves have rejected the law of God by our own conduct.