I think you said a lot by calling sex a "tool", though. It's got to serve the story. I know a lot of writers will slap on the "PWP" label on stuff that's short and graphic. But to me, if the story's got a point to make, and the sex helps make that point, it's not just gratuitous smut. However, if it truly is just "oh, here they are, let's get it on", then I do lose interest. I've just read too much of it.