What does Matthew 5:27 mean?

Jesus is giving examples to back up His statement in verse 20 that nobody can enter the kingdom of heaven unless their righteousness exceeds that of the scribes and Pharisees. In part, this is meant to suggest that those groups were hypocritical. More immediately, it was meant to emphasize the level of perfection demanded by heaven. Christ's first example connected the commandment against murder with God's expectation not even to lash out with insults in anger or risk the hell of fire (Matthew 5:21–22).

Now Jesus says that His listeners have heard, "You shall not commit adultery." This is the seventh of the Ten Commandments (Exodus 20:14), taught to every Jewish person by their religious leaders. When Jesus uses the "you have heard" phrase, He does not mean to imply that the statement is false. Rather, He follows those statements with a more accurate understanding. Jesus does not go on to contradict the law against adultery—He magnifies it.

As he did before, Jesus will point out that it's not "good enough" to simply avoid physical sin. What we think in our hearts leads to what we do with our bodies—so honoring this commandment means something more than not having sex outside of marriage. God's intent is for people to control what happens in their hearts and minds well before the opportunity for physical adultery becomes available.