Charles Spurgeon Commentary Matthew 19:9

"And I say unto you, Whosoever shall put away his wife, except for fornication, and shall marry another, committeth adultery: and he that marrieth her when she is put away committeth adultery." — Matthew 19:9 (ASV)

Fornication makes the guilty person a fit subject for just and lawful divorce, for it virtually annuls the marriage bond. In a case of fornication, upon clear proof, the tie can be broken, but in no other case.

Any other sort of divorce is, by the law of God, null and void, and it involves the persons who act upon it in the crime of adultery. Whoso marrieth her who is put away doth commit adultery, since she is not really divorced, but remains the wife of her former husband.

Our King tolerates none of those enactments which, in certain countries, trifle with the bonds of matrimony. Nations may make what laws they dare, but they cannot alter facts—persons once married are, in the sight of God, married for life, with the one exception of proven fornication.