Why does the idea of my bf cheating on me secretly turn me on?

There is no meaningful answer to your question, if that's what turns you on then that's what turns you on, why is not an issue. What you can do is explore the fantasies and try to determine if there's a way to incorporate them into an otherwise normal life, any or paraphilia is only considered a problem when it interferes with your life outside the bedroom. I'd suggest, if this really bothers you, that you try talking to a counselor or therapist, it may well be a symptom of other problems, and addressing those may solve it.