“You have heard that it was said, ‘Do not commit adultery. But I say to you that everyone who looks at a woman to lust for her has already committed adultery with her in his heart.
This sermon on the mount was not given so that we have new rules to follow but to show us that it is impossible for us to fulfill God’s law.
Jesus in the sermon on the mount is describing God’s standard, which is a standard about our heart because this is where God dwells. That standard is impossible for fallen man to meet, but we need to know what it is so that no one is not ashamed or thinks they are pleasing to God. This of course sets us up so that only Christ’s righteousness can be pleasing to God and only Christ in us can be a temple in which God dwells.