Jesus says, insofar as a man thinks of adultery he has committed adultery.
One would respond to this, as I know I did, “then it is impossible to not commit adultery.”
This I believe is the trick of the phrase… We can spend our lives trying to control our every thought, and suffering through our every day temptations; and we can cause others who commit adultery to suffer in the name of justice… We can shame them, ridicule them, shun them, cast them out, and in the times of Jesus, kill them for adultery. Or, we can do as Jesus truly intended us to do when we realize the absurdity of his claim; have compassion on those who fall to the temptation.
Jesus is saying, “We have all thought of committing adultery. The sinner has merely acted upon what we all desire.”
We are to then recognize, we are not free from the same lusts as the adulterers, so we should have compassion upon them; because were we lusty enough, and the opportunity arose, we would partake. So, don’t cast stones at others for the sins you desire to commit. Every person who is healthy desires other men’s wives, or other women’s husbands. That is natural. Don’t hate the people who have desire, and feed that desire.