There are some interesting comments in the following discussion about the fact that when watching porn your ability to think morally is effectively turned off. Maybe that is the reason that while watching you are ok with the process, but after you stop you feel disgusted with yourself for what you have just done. Once you stop watching and are no longer aroused your moral reasoning switches back on.
Do you think it is a wise thing for a disciple of Christ to be doing something that limits your ability to make moral decisions?
Get Off the Roof! 