It's the "sin in your heart" part that really gets me. One is tempted. One chooses not to act on that temptation. You've still sinned! So much for free will.
I will not be help responsible for every stray thought that enters my head. I will be responsible for encouraging those thoughts and acting upon them.
I also wonder, why does that only apply to adultery? I've seen/heard people that have gotten me so angry, the thought of shooting them in the face has crossed my mind. Therefore, I have already murdered in my heart!
Gah, this is why I'm studying philosophy and moderate Buddhism. The end goal is to work out aggressive and debilitating thoughts, not give up and start apologizing to Gawd.