Note 6 at Mt 19:9: Although adultery denotes one who has unlawful intercourse with the spouse of another, in its much broader definition, it means to violate or pollute. Israel violated or polluted their covenant with God (Isa 57:3, Eze 23:43, and Ho 2:2). Jeremiah said, "she [Judah] defiled the land, and committed adultery" (Jer 3:9, brackets mine).

Adultery, when used 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 the introduction of another party and relationship into the marriage. "They two shall be one flesh" (Eph 5:31, emphasis added).

Divorce also adulterates or pollutes marriage because it disrupts or denies the divorced parties the right to be faithful to their covenant of companionship. Anytime people divorce their mates (except for fornication) and marry others, they are guilty of adultery. They have polluted and destroyed relationships intended by God to be permanent and pure.

Why the "except for fornication" clause? Jesus was not saying that if fornication is involved, the couple must divorce. 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 extramarital relationship has already polluted the marriage vow.