In my first two novels, I didn’t include a lot of sex. It wasn’t appropriate to the story, and it seemed like a bad idea as a focus while running from zombies and cannibal cultists.
My current novel is literary fiction with a lot of drug use. Sex scenes are a clear and obvious elements in this book. I’ve tried to keep the sex scenes somewhere in the not too explicit range, because of the themes of the book itself (it’s about false accusations and isolation/loneliness).
Sex scenes aren’t something I won’t include; they are in fact, something I enjoy writing, but only if it makes sense. I have a few books in the planning stages that will be more explicit. For example, I have a planned book that is a vampire love story. There’s absolutely going to be sex in that book – it only makes sense in context.
It’s a strange part of the job, how explicit is too explicit? Go too far, and you are writing erotica, don’t go far enough and it comes off as weird. Also, Guy Gavriel Kay, a great writer, tried to use slang and sex in his one attempt at modern day horror. Unfortunately, he did so without being familiar with colloquialisms and kept misusing the term sixty-nine. He meant blow job, but he didn’t know they weren’t the same thing. If you screw up sex scenes, it can come across as the exact opposite of what you want to achieve. Instead of adding a salacious thrill to the book it can detract from the story, pulling your reader intellectually out of the narrative.