Questions, Questions

What's the social etiquette for doing a little fantasizing about a friend's girlfriend or wife? The closer you are, the more shame you should feel? Should there be any at all - I'm not talking about stalking or carrying a torch, I'm talking about during an onanism session letting her walk into the scene, and you're like mmmm, she's hot...? I mean, your friend obviously thinks she's hot, why wouldn't you? Lil harmless fantasy, that outta bounds?