Actually, the better question may be "How Much is Too Little?" I have noticed two interesting trends in gay fiction and I imagine it holds true across mainstream fiction as well:
First, there is the trend toward separating well-written fiction with explicit sex scenes into its own category: "erotic". I imagine this began as a way to flag the reader that he/she was stumbling into some frisky ground. I understand that. At some times in my own life I have not wanted to have to stand in someone's bedroom in the middle of a scene: it felt invasive. I was embarrassed. For me and for them. A warning is understandable. But my concern is that in our zeal to pass on a polite warning we have relegated some high quality fiction to a seamy back room.
The second trend I notice is that in a well-written book or story where the reader would benefit from more detail in a sex scene, the writer shies away. At times, one can almost feel an author holding sex at arm's length - his or her own squeamishness getting in the way of the integrity of the story. If you think about it, sex is part of human experience. It is such a deep part - and a necessary one - that how can one hope to paint a true picture of human experience without exploring a character's sexual viewpoint?
I just recently read a really amazing novel, full of truly beautiful language, imagery, characterization - everything a great book needs and a great writer boasts. But where a sex scene should have been was a paragraph of euphemisms and a hasty retreat to the following morning. The thing is, this scene was pivotal, and what happened in that bed mattered. Details mattered. It mattered to the state of mind of the main character, because that event changed his life. If the author had been courageous enough to add more detail, the scene could have been spun into a haunting one that would have informed the story on a deeper level and enriched the entire novel. It was good enough as it stood, but could have been so much better because it could have made the novel better quality story-telling.
There is for most of us a difference between "erotica" and "porn". To me there is. It's quite simple: in pornography the explicit scene is about the sex, as is the whole piece. It is offered for the reader to revel in the detailed mechanics; it assumes that the reader isn't overly invested in the characterization or plot. In erotica/erotic fiction, the scene informs the story in the rest of the piece - the rest of the piece being of the same quality as any work of fiction worth its name. Porn examines the technical detail of the sex act; erotica examines the emotion, physicality, the story as a whole, with the details of a sex act being only one more layer of meaning amongst the many offered. Perhaps writers avoid writing sex scenes because they assume others will see them as writing pornography. Perhaps we as writers need to speak up more about the difference, and the merit of a good graphic sex scene.
I believe that we as a society, and therefore the literature that reflects our collective psyche, are evolving. I see a day when a writer won't think twice about including graphic sex in a story within the context of the story - as naturally as human sexuality occurs within the context of a life. Sexuality is so integral to our core, to omit it when it is part of our story, seems to me a real disservice to a good book.