The Pharisees wanted to know whether they could divorce their wives for any reason. Jesus explained that was not God’s original intention. Therefore, Jesus had to explain why Moses did allow divorce, but then the Lord Jesus declared His position,

“And I say to you, whoever divorces his wife, except for sexual immorality, and marries another, commits adultery; and whoever marries her who is divorced commits adultery” (Matthew 19.9).

If a man divorces his wife and marries another, he commits adultery, and the reason it is “adultery,” is that technically he is still married to his first wife. The Lord gave as the only exception to that rule when sexual immorality has taken place, that is, if a man puts away His wife because of her sexual immorality, then he may marry another.