"How much pornographic imagery in a movie is "acceptable" for a Christian? In other words, at what point does a Christian make the difficult decision not to see a movie due to its graphic sexuality? Ever? Two sex scenes? Ten? Only one?"
Each Christian must make their own decision. My standard is any graphic sex and I won't see the movie. Do the images improve my relationship to God or hurt it? Do they help my marriage or hurt it?
Philippians 4:8 is a good standard:
8 Finally, brothers, whatever is true, whatever is noble, whatever is right, whatever is pure, whatever is lovely, whatever is admirable—if anything is excellent or praiseworthy—think about such things.
I appreciate your sensitivity. It's uncommon to find Christians who choose not to see a movie because of some of its imagery.
I frankly *did* want to see 300, but after I learned what was in it, I had to make the difficult decision of staying home instead.