Although adultery denotes one who has unlawful intercourse with the spouse of another, in its much broader term, it means to violate or pollute. Israel violated or polluted their covenant with God. Jeremiah says, ‘She (Judah) defiled the land, and committed adultery’ (Jer. 3:9).
Adultery in relation to marriage also reflects a violation of the covenant of companionship. Sex outside of marriage is adultery because it violates or pollutes the covenant of marriage by introducing another party and bringing that relationship into the marriage. ‘They two shall be one flesh’ (Eph. 5:31).
Divorce also adulterates or pollutes marriage because it disrupts or denies the divorced parties the right to be faithful to their covenant of companionship. Any time one divorces his mate (except for fornication) and marries another, he is guilty of adultery. He has polluted and destroyed a relationship intended by God to be permanent and pure.
Why the ‘except for fornication’ clause? Jesus is not saying that if fornication is involved, we must divorce, but rather, when fornication is involved, that is the only time when the person getting the divorce is not guilty of adultery. That is because the partner who had an extra-marital relationship has already polluted the marriage vow. In God’s original design for marriage He intended marriage to be sacred, precious, pure, and permanent. The marriage covenant represents a final, irrevocable commitment where the man and the woman renounce the right to live for themselves and become ‘heirs together of the grace of life’ (1 Pet. 3:7).