Sometimes it seems that a modern novel isn't complete without a sex scene, sometimes quite a few. Writers seem to feel pressured into including some quite graphic details, often with disastrous consequences. The physiology and mechanics of the act itself are rarely critical to the plot; the key feature is the nature of the relationship between the characters. Is it tender? Is it mutually consensual? Is it part of a permanent coupling or merely a one-night stand?
Personally, I think a good sex scene should have two key components. First, the reader should be able to relate to the event – a seventeen-orgasm bonk is just a joke. I think the scenes are actually sexier where the individuals are themselves more ordinary – an extension of the Brief Encounter idea, where the very ordinariness of the Celia Johnson and Trevor Howard characters is what makes it work. The reader wants to relate to every part of the book. Athletic male six-packs and female catwalk figures don't quite do it.
The second aspect of a good sex scene is that less is more. Alfred Hitchcock always maintained that the human mind was more afraid of what it could imagine than what it actually saw; I think the same applies to sex. Let the reader fill in the blanks for themselves – in any case, who are you, the author, to presume that you're the expert?
Finally, a strange thought arose today from Emma Baird's blog. She's currently writing an LGBT novel, and she posted a sample chapter which referred to the sexual awakening of one of the characters. Both of the characters are men, so we have to assume that Emma doesn't have a lot of personal experience. I've met authors who have passionately argued that a woman can't possibly write from a male point of view, yet I found her handling of the scene to be tender and appropriate – perhaps precisely because she had to leave so much to the reader's imagination. Is it easier for straight writers to write LBGT fiction about the other gender? It's an interesting question.