I'm another newbie to the whole field, but in my experience the voyeurism/compersion distinction works something like this: I can see my boyfriend's happy after a date with another woman, and I'm happy about that. That's compersion. Then, with a wink and a grin, I ask for details, and if he gives them (with the other woman's permission, explicit or implicit), I usually get turned on to some extent. That's voyeurism.
I've only watched him with another woman once, but that experience I can only describe as an inextricable jumble of compersion jollies and voyeuristic jollies. I don't feel a strong need to sort out what's what: it was jolly, and that's the important thing, yes?