Does the forbidden fruit really always taste the sweetest?

Honestly, I'm not sure. I read Sam/Dean slash from Supernatural and thinking about it, the only reason it doesn't squick me is that what I'm imagining when I'm reading those stories are the actors; I can enjoy the fic and the kink of them being brothers as characters but I'm always aware that they aren't really brothers, they're just unrelated actors. If I was reading real-life slash then it probably would squick me.