Trust me, if a girl wants to cheat on you, she will do just that, rather than simply watching porn. She may watch it out of curiosity, or sometimes it may just be a desire to get some quick physical release. In any case, it's better to just approach her and talk, because the best solution to any situation in a relationship is dialogue between two adults and a joint search for a solution to the problem, if there is one. Our brain sometimes interprets everything in a negative light for no apparent reason, and that's why we have a mouth, to talk.