Matthew 5:31-32 KJV
31 It hath been said, Whosoever shall put away his wife, let him give her a writing of divorcement: 32 But I say unto you, That whosoever shall put away his wife, saving for the cause of fornication, causeth her to commit adultery: and whosoever shall marry her that is divorced committeth adultery.
The seventh commandment is "Thou shalt not commit adultery." Here Jesus is taking it further and explaining how one causes another to commit adultery.
God hates divorce. Marriage is a covenant bond that is never to be broken. God desires purity and when a divorced person remarries, the purity is already gone.
I know this is a big issue and most people don't think it is fair, but purity is a big issue with God as is a covenant bond.