The answer to this question, IMHO, really depends on the how and whether the individual author feels comfortable about turning the spotlight onto this side of his/her human experience. The old adage is "write what you know," right? So, in many ways the writer is drawing on his/her own knowledge of and experience with this most personal and intimate of human interactions. Do we really want to bare ourselves (literally, in this case) in such a way? What if our mother-in-law reads the scene? Will she look at us funny forever after?
Then, too, there's the question of performance anxiety. As I noted in my last post, writers are inherently competitive. I remember being slightly offended when another author who was critiquing my MSS thought the sex "wasn't all that hot" (really? what would it take to turn this person on, one wonders). No doubt about it, a lousy sex scene can really sink a book. Boring, hum drum, coy, prudish, weird, and saccharine are all dreadful possibilities if you don't get it right. Eek. No pressure.
Then there's the question of satisfying the reader. Does he/she like it hot? Or is he/she the type who prefers intimations over intimacy? How much do you put on stage, as it were, and how much takes place behind closed doors that the reader never gets to open? Is the reader going to feel unsatisfied or overindulged? What's the sweet spot (no, the metaphor refers to a tennis racket. Really) where good taste and good fun combine?
Really, it's a lot to consider, and I'm not even going to tackle the questions of personal morality, religiosity, inhibitions, and cultural norms. The author's walking a tightrope above all of these. No wonder many have such a hard time when it comes to engineering our characters' romantic lives.
Here's the thing, though: sex is part of what makes us human. Pretending it doesn't exist doesn't work, either from a family planning perspective or from the novelist's. I'm not saying it should be in every book, any more than there should be a fire-breathing giant spider in every book (as much as I'd like to insert such a monster in some boring stories). I'm just saying that neither the reader nor the writer should make a hard and fast rule about either including or excluding it. If it's right for the story, it should be there. If it doesn't move the action, plot, or tension along, it's in the way. (I'm assuming, by the way, that we're not talking about erotica, which has its own conventions.) When it's done right, it feels genuine, emotionally engaging, and logical all at the same time.
Tall order? You bet. Have you done it? No, I mean, have you written sex in fiction? (Sheesh.) What worked and what didn't. I'd love to hear. Peace.