Church Fathers on Matthew 19:9

Having stopped their mouths, He now set forth the Law with authority, saying, 'But I say unto you, that whosoever shall put away his wife, except for fornication, and marrieth another, committeth adultery.

Perhaps some one will say, that Jesus in thus speaking, suffered wives to be put away for the same cause that Moses suffered them, which He says was for the hardness of the hearts of the Jews. But to this it is to be answered, that if by the Law an adulteress is stoned, that sin is not to be understood as the shameful thing for which Moses suffers a writing of divorcement; [Deut 24:1] for in a cause of adultery it was not lawful to give a writing of divorcement. But Moses…

It is fornication alone which destroys the relationship of the wife; for when she has divided one flesh into two, and has separated herself by fornication from her husband, she is not to be retained, lest she should bring her husband also under the curse, which Scripture has spoken, 'He that keepeth an adulteress is a fool and wicked.' [Prov 18:23]