As Allen engages in the discussion, his face changes. What seemed like a small head-rush of excitement is now turning into compulsive lust.

What is Allen experiencing here?

Have you ever experienced that "I crossed the line anyway" or "I don't care about the consequences anymore" feeling? 

Where do you cross the line from doing something you find exciting (without loosing your head) to a place where you  go forward in lust without thinking about consequences?

Is that line even visible?