Matthew 19:8-9

8 Jesus answered them, ‘Moses said this because you people did not want to obey God. But it was different in the beginning, when God made the world. 9 But now I say to you: A man must not send his wife away and marry another woman. If he does that, it is the same as if he had sex with another man's wife. A man may only send his wife away if she has had sex with another man.’