Then you really go to town behind their ears and the sounds they make as they lean into make you understand that no amount of self-manipulation equals the real thing.
If you were given a choice between jerking yourself off and getting a no-strings-attached blowie, what would you choose?
Edit: I'm fascinated at the amount of downvotes this comment is gathering, especially in a fluff comm like this. Anyone want to tell me why they're downvoting? Because sex bad?