I fantasize about cheating on my bf all the time
That doesn't sound good, why are you fantasizing about cheating on him?
I don't know! Must be something about the forbidden.
It's understandable that at times we can't control what you fantasize about but it's still wrong to cheat. If you are thinking of cheating then you might want to talk to your significant other about these feelings.
It's not a real person. You can't hurt anyone by just having fantasies.
I understand, but continuously fantasizing about cheating can make you end up acting upon those fantasies as you are normalizing the behavior in your head and not thinking that it is wrong. You can spice things up your relationship with your boyfriend in bed with suggestion more exotic sexual activities so that you don't think about cheating.