“‘you shall not commit adultery.” (Deuteronomy 5:18, Exodus 20:14 ESV)
One can fulfill the external requirement of this commandment (and probably feel self righteous about it), but can utterly fail the internal, heart requirement of it – lust. Though this commandment was about a man having sexual relations with a woman who is married to someone else, the broader context and teaching of the bible makes it clear that sexual purity (external & an internal purity of the heart) is what God is calling us to. We should never long for & desire (lust after) anyone that we are not in a covenantal relationship with. This gets to the heart and trusting God with the marriage that He has placed you in. What fuels your fantasies? Jesus unpacks the original intent of this commandment in Matthew 5:27-30 and the drastic measures He calls us to in killing it in our lives. Staying free from the external act of adultery, but burning with lust within is not freedom of heart.