Sex Scenes in Books - When to Use Them

Something I ponder frequently is if and when to narrate sex in books. When should I explain exactly what the characters get up to, and when should I leave it up to the reader's imagination?

Fortunately, thanks to the recent erotica boom, it is now much easier to make our decisions based on what's best for our books, rather than having to shy away from graphic sex so as not to shock the average reader. Obviously in depends on our audiences, but we are much more free to pluck from the fields of...