fearbeforex , 22

I have been dating my boyfriend for four years now, and over the course of time we have graduated high school, begun college, made new friends, lost old ones, and survived many other milestones in our life. Our relationship itself..it's perfect. We hardly ever fight, and when we do, it's resolved quickly and efficiently, and let's just say that I could not be happier in general. However, over the course of the past year, he has neglected to hide the fact that he masturbates to pictures of one of our old best friends in her prom dress (YES, her prom dress! Granted, the top is quite small, but she is 100% covered up and smiling). The first time it happened I was speechless, the second time, hurt and distrusting, and the third, complete anger. And him? Ashamed, ashamed, and ashamed. This problem in and of itself not only hurts me, but makes me feel like it should cause a giant disruption in how much I trust him and how little respect he has for me, because no matter what the problem is, it should not take three arguments for him to respect the fact that it hurts my feelings and he needs to stop. At the same time, I realize the whole thing is probably one big fantasy, and I am practically indifferent to the fact that he looks at other girls because I know how much he loves me. Is this as disturbing as my mind is telling me it is, what do I say to make him understand, and if you were in this relationship, would this be a make it or break it deal?