A lot of the "sin" involves the mind of the person who is committing the act. Rok and I view porn; In our minds, we are not guilty of wrongdoing while Christians may feel that we are. However, as Aineo has rightly pointed out, there are no criminal penalties for viewing porn in the privacy of our own homes.
However, take the act of stealing. Is stealing always a crime? Christians might say, "Well, of course it is." But I say different. Here's why in anecdotal fashion:
Suppose my family is hungry and I come to you (generic 'you', Aineo ) for some bread. You tell me that I can't have any of your bread and you send me away. The next day my family is still hungry, so I return to your door and ask you for some bread to feed my family. Again you refuse and send me away. The third day I return because my family is still hungry. If you tell me no, you'd better keep a sharp eye on your bread, because given the circumstances I'm going to feed my family. You call the stealing a sin; I call the greed a sin. It's a matter of perspective.
Same scenario applies to killing. If someone breaks into my home and is threatening the lives my family, given the chance I'll kill the intruder to protect my family. Is killing then a sin? In the mind of the person committing the act, the answer is no.
The whole idea that sin is black or white and never gray cannot sustain itself due to examples like the one I've given. To say sin is absolute is hyperbole in its highest form.