Here’s the problem: A sex scene, like every other scene, has to both be necessary and move the plot. If you don’t really know the characters and how they feel at that moment and you won't till you get to know them better, and maybe have to modify the personality and motivation you expected them to have, you’re in danger of simply writing out your own fantasy and focusing on plumbing rather then emotion. That can be fun, but may not be more then that so far as the novel is concerned.

Jennie Crusie has an interesting view, which is that like any other scene, it’s much more interesting if things don’t go right.

I tend to feel that it should either represent an emotional pivot-point that changes at least one of the characters in a significant way, or be an utter disaster that does that same thing.

Otherwise it’s just a bit of erotica. Fun, perhaps, but no more important to plot movement than being with the characters as they talk about nothing special over dinner.

In romance sex scenes play a big role in the plot. It's the next step in the relationship. How detailed the scene is is up to the author, and what works for the characters. I've read books that are very detailed, and others that get the characters started then shut the door on the reader. Either way, as long as it isn't forced, and the characters have reached that point in the relationship, then have fun with it, and if it makes you blush, then all the better.

• I don't get that. Why does everyone keep saying that everything HAS to be important and or move the plot? Why CAN'T a scene just be there for the fun of it?

Well, this is a bit off subject, and about the question asked. It’s also longer then I intended, and about not bedroom fun, but…

At its most basic, a story is about happiness. Someone who has a stable situation had something happen that upset it. It might be that a new employee, who moves the protagonist to anger every time they meet has just become a permanent part of his life, like it or not. It might be that s/he’s lost at cards, or has purchased a boat they think will be great (but we know it won’t be that easy). Maybe the character’s car breaks down outside a Transylvanian castle. It’s the inciting incident.
 
So now the protagonist is unsettled, to the point where the focus of their life is solving what they view as “the problem.” Their view of what that problem is may change over time, but not their dedication to resolving the issue. And it quickly grows to the point where they have to solve it, and giving up isn’t an option. This brings tension and conflict, something a reader feeds on. In romance it’s sexual tension, but tension is tension, and a reason for the reader and the character to worry.

Worse, bastards that we are, we keep tripping the protagonist. Again and again s/he tries to solve the problem but always fails, in a way that makes things worse, and keeps them—and the reader—saying, “Damn, what do I do now!” There’s nothing a reader likes better than a reason to worry. They feed on it.

The classic adventure story, pioneered by Edgar Rice Burroughs, took a protagonist or a group of people and got them into worse and worse trouble, till it was certain they would die. Then, he’d abandon them there, and jump to another situation, leaving those people to certain death.

He would then go back to the first group, rescue them, and then screw them again, to go back and rescue group two. It was, literally, the cliffhanger, and it sold a lot of books, because readers love disaster. You’ll see that technique active in films like the Star Wars series. But suppose, in the middle of one of the Star Wars films, they had followed Luke Skywalker as he and his friends landed by a carnival and enjoyed the rides and an evening of fun. Wouldn’t you have been saying, “But what a minute…how series could all that other stuff be if he can take the night off to relax?” Doesn’t that apply, as well, if the character takes a night off for sex that we share that’s unrelated to the story?

Orphan Annie, the comic strip, was popular for many years because no matter what she did, disaster followed Annie. No one wanted to read about the nice birthday party she attended, unless the cake had a snake inside for her damn dog to kill.

Can scenes be fun? Hell yes. People need a break from pressure, to keep the story from becoming melodrama. But still, every line in the story needs to matter. It needs to set the scene, develop character, or move the plot.

Thus, in a story I call Posse, at one point the protagonist, a policeman, has gone dancing with the lady he fancies. He’s having a great time, and as he gets back to his seat he thinks, Damn, this lady can dance. And that’s good. But a moment later he sees someone by the door who tried to kill him. And in doing so we not only get back to tension, the fun he had been having creates a greater contrast and shock as tension reenters. Did I have to show him talking, drinking and having fun over the evening? No. That would slow the narrative. I spent some time showing their meeting because that developed character and showed them reacting to each other. But in that section I had someone come by who mentioned something that moved the plot a bit, so as to not lose focus. The trip to the club and getting them on the dance floor took a paragraph, about a third of a page. And that’s all that was needed to set the scene.

You don’t want a single moment when a reader might wonder if there are tasks that need doing in their own life. You want them to keep reading because they need to. End a chapter with no problems in play and the reader just might decide to take a break, and never come back.

• I think Marcie makes a good point. If something is really burning to get written, get it down.

Once, I was driving home, when the Association’s song, Windy, came on. And as the first line was sung: “Who's peeking out from under a stairway,” I had a vision of a fairy, golden hair and translucent dragonfly wings, perhaps nine inches tall, peering out of such a stairway in a neighborhood of tenement apartments.

It was a cute notion, but then my engineering background kicked in and said she needed minimum weight to get off the ground, so my little darling had to be nude, which meant I needed someone human to see her and appreciate her beauty.

By the time I got home, the scene was complete. Poor Jonathan Trent was hung over. He was finished with the brown-whimpers phase that came with the dawn, and was currently in the “I can walk to the train if I don’t make any sudden changes in direction,” stage, with his eyes bulging only moderately against the inside of his skull. And then he sees the sprite peering out from under the steps of a tenement apartment building, her hair a flick of captured sunshine against the age-darkened stone.

The scene absolutely had to be written, and was great fun to do. But no story came with it, so after Jon heads off to work, convinced that it must have been a hallucination, that was that and I put the piece aside.

Two years later I had my WIP protagonist, a rather nice lady, stranded in a world far from Earth, and in need of a partner for a quest she was about to take on. I was stumped, till Jon came to mind. Who better to transport him to meet up with Colleen than the fairy?

And just like that, the scene I wrote because it needed to be written became chapter one of the novel that needed him as badly as he needed to be created.

So, be the scene one from the story in progress or simply one that appeals, write it. And if it isn’t of immediate use put it in your possibilities box. Maybe it'll come in handy, later. Who knows? Certainly, a gift from a muse should never be ignored.