Found While Looking for Something Else

The Lord says this: "Everyone who divorces his wife and marries another commits adultery, and he who marries a woman divorced from her husband commits adultery." [Luke 16:18] 

You, however, divorce your wife as if you had every right to do so and you are not afraid of doing her an injury.  You believe it is permissible because the natural law does not forbid it. 

But the law of God does forbid it!  And you, without being disobedient to human law, ought also to fear God.  Listen to his law:  "What God has joined together, let no man put asunder." [Matt. 19:6] 

It is not of any great importance whether you commit adultery openly or under the guise of 'marriage'.  There is one difference only:  the fault committed on the pretext of principle is more serious than that which is committed secretly. 

-- St Ambrose
On the Gospel of St Luke, 8, 2 (PL15, 1765)

Found in Drinking from  the Hidden Fountain: A Patristic Breviary, Ancient Wisdom for Today's World,  by Thomas Spidlik, Cistercian Publications, 1993.