I think it depends on your market – as people say above, some folk read fanfic for the porn. If you are writing a novel on the other hand, I would avoid gratuitous sex scenes - if it forms a natural part of the narrative or is relevant to the story you are telling then go for it, but if it is being shoe-horned in for the sake of it then it will always feel awkward and out of place no matter how well written. The amount of detail is up to you, but less is often more. These are my observations as a reader more than a writer, i have yet to perfect writing anything :D