i disagree with this other comment as a man i know the fundamental truth about why men watch porn and its for 3 main reasons one was what Average guy said. the second reason is release of stress although it may not seem it masturbation creates a chemical reaction in men that helps release pent up stress (namely testosterone) and also helps prevent prostate cancer which seeing as he is your fiance if you are trying for a child at some point in the future you do not want to happen. the third reason could be because he has certain fetishes he cant reveal to you because he feels that you will either not preform or does not want to force you to do (this group also tend to use imagination to picture their partner and them doing it rather than the woman). as Average guy said watch it with him but ask him why he finds it sexy and if its something you can accept and maybe do with him do it for example if its a bit of cosplay buy an outfit he likes and tease him with it. nothing turns me on more than my lady looking at my fetishes and telling me what she could do to make it better and if he is like me he will be eating out of your hand.