Matthew 5:31-32

31‘God's Law also says: “Any man who wants to send his wife away must give her a letter. This letter must say that they are no longer married.”

32But what I tell you is this: A man must not send his wife away except for one reason. He may only send her away if she has had sex with another man. If he sends her away for any other reason, he has done a wrong thing. It is like he has caused her to have sex with another man. Also, if a woman has left her husband, another man must not marry her. That is the same as if he had sex with another man's wife.’