'Unfaithfulness' in the Bible

but I say to you that whoever divorces his wife, except on grounds of sexual immorality, causes her to commit adultery; and whoever marries a woman who has been divorced commits adultery.

And I tell you that whoever divorces his wife for any reason except her unfaithfulness, and marries another woman, commits adultery."

What if some of the Jews were unfaithful? Their unfaithfulness cannot cancel God's faithfulness, can it?