In Matthew 5:27-30, the Lord Jesus gave the second contrasting, corrective illustration between the rabbinic teaching the people heard wrongly presented to them and His interpretation of the Law.  In this section, He spoke about adultery, making it clear that the sin is a lot more than just a physical act.  He taught (as He did with murder) that the sin of adultery begins in the heart.  The religious rulers, as with most people today, needed a divinely given course on the doctrine of sin.  They had lowered the level of God's holy standards to an externalism they felt they could achieve, and then they self-righteously convinced themselves God was pleased with their great efforts to master their own standards of "righteousness".  They were pious-acting hypocrites!