You are friends. You both have an interest in movies. You have casual sex. Neither of you is romantically interested in each other, you are just a pair of friends who sometimes have sex because it feels good. So why would it suddenly be problematic to learn that he has a fiance soon to be wife? Everything in your relationship is still the same, his fiance just knows that you are another movie buff, nothing about the other activities you do together. No need to feel guilty about having sex with him.