As I understand it, adultery is where at least one person is married (but obviously not to the person he or she is having sex with.) But it is still sinful or at least unholy and would still be considered contradictory to Christian faith. Yet, some Christians, mistakenly in my opinion, take that Biblical quote about adultery and extend it to anything sinful. So, if I were to give serious thought to stealing from a store, they would consider me to have commited theivery of the heart...