Masturbation in Marriage

My wife and I love each other and wouldn't cheat on each other given the chance. Having said that, I have had sexual fantasies with actresses or other people ever since I can remember. Sometimes I feel tempted to masturbate to a photo of someone else or while thinking about these fantasies. I have a good sex life with my wife, but I always feel this urge. I don't know what's acceptable, especially as an LDS priesthood holder, and it feels awkward to tell my wife about this. I don't want to test the limits, but I don't want unnecessary limits either.