There are two ways you can commit adultery within a marriage. The first way is the obvious one. You are having an extramarital affair while you are still married. That is adultery. The second way is the way Jesus described it.
It goes like this. If you see someone else you like, you divorce your wife or husband so you can be with that person and marry them. That is also considered adultery. That is why, in Matthew, when Jesus was speaking, he said:
Matthew 5:32 NIV
But I tell you that anyone who divorces his wife, except for sexual immorality, makes her the victim of adultery, and anyone who marries a divorced woman commits adultery.
You make her the victim of adultery because your divorce was specifically designed so you could be with someone else. That is also adultery. Now, not all divorces and remarriages are adultery, but if you are getting divorced just so you can marry someone else, that is adultery as well.
You cannot use the legal cover of divorce to hide adultery.
