He says it 4 times in the Gospels. Shocked heck out of the Apostles.
Strong's Greek: 3430. μοιχεία (moicheia) -- adultery
Jesus says that the exception would be if the original union were in itself sexually unlawful [different word: porneia.]
Strong's Greek: 4203. πορνεύω (porneuó) -- unlawful sexual intercourse [as distinguished from moicheis-adultery]
This is not some distinctively Catholic thing. All of Christendom understood it this way until quite recently, based on the very words of the Gospel. It's difficult, but we're stuck with what He said.
No I am not mistaken...Jesus says if you divorce your wife it is adultery UNLESS the divorce is due to fornication with someone else while you are married...
It has nothing to do with you finding out your wife was not a virgin when you married her...
Besides this rule applies to those under the Law, not Christians under grace...