There are a lot of ways to mess up a sex scene, believe it or not. At first glance, it probably seems relatively brainless to write. After all, everyone knows what's going to happen. We know what goes where and thinking of a few creative ways for the hero and heroine to use the location or toys to spice things up isn't rocket science. But it's not that simple, and I'll explain why.
There are a two main factors that go into making a sex scene special, and I'll dive into all of them, but I'm going to start with the most important, and the most often forgotten.
1.) A good sex scene shows growth or change in the relationship between the hero and heroine.
For example, in one of my earlier books, Punished, the heroine has never had an orgasm with a man before. She gradually realizes she is extremely turned on by BDSM (and Logan Steel, of course, the hero). In each sex scene, we see her development start to shift. One shows her change of perspective, realizing that sex can be enjoyable. There's a distinct change that takes place after this scene. She's a slightly different woman now. She has had a taste of how amazing sex can actually be, and that realization starts to influence her decisions. As the book goes on, she starts to develop down an almost dark path, craving the pain of BDSM for unhealthy reasons, because they are helping her to cope with her bad breakup. It gives her control over something she never had control of in her previous abusive relationship. The sex scenes then help develop this dynamic between the hero and heroine, and we also see how Logan's own reliance on BDSM to get off shifts as he sees that he can be happy with purely vanilla sex as long as it's what is good for the heroine.
The short version is that much of the plot and character development in Punished depends on the sex scenes. They aren't just steamy and fun. They are essential to the plot of the story. You couldn't cut them out and read what was in between and have it work as a story. You'd be confused, not understanding why the characters were seemingly changing without explanation.
If you write or read a story where the sex scenes can be omitted without harming the story, chances are, those sex scenes aren't doing what they are supposed to.
To give one more example in a non-BDSM kind of story. Take your average romance. Guy meets girl but for some reason, they can't be together so easily. Let's say he is the typical bad boy and she is a good girl. Maybe she has a daughter and she's attracted to him, but worries she could be bringing a bad influence into her daughter's life. The sex scenes would be a great way to start showing there was something more to this guy. Maybe he's tough and rough around the edges on their dates, but he shows a tenderness in the bedroom that makes her start to see him in a different light. It can be that subtle. Maybe just the way his rough, hungry kisses turned tender for only a moment. The way he brushed her hair away from her forehead and tucked it behind her ear, etc. Something that small can make her start looking for the softer parts of him. It can also be the first stepping stone showing the hero that she's different. She's special, and maybe she's going to be the reason he changes his ways.
Now, all that being said, I think it's worth mentioning that romance comes in a lot of flavors. There are definitely times when you might be in the mood to read something dirty and fun without all the weight of character development and heavy plots. There are authors who write exactly that kind of story, too, and they do a great job of it.
My advice and thoughts are more targeted toward something closer to the traditional steamy romance that has anywhere from one to four or five sex scenes but also aims to have character development and some level of plot development.
2.) A good sex scene brings the characters into the scene. As a general rule of thumb, it shouldn't be possible to just copy paste a sex scene from one of your books into another one without causing some major problems. Why? Because in general, readers aren't interested in just reading that a penis went into a vagina. What makes a sex scene satisfying is hearing how the characters are reacting uniquely to it based on everything we know about them. There should be dialogue, and it should build from what has come before in the story. Is she a baker? Maybe she should make some kind of ridiculous but funny baking metaphor at an awkward time. Or maybe it's more subtle, like the way she focuses more on his hands because being a baker requires strong hands for kneading all the dough.
One way or another, the sex scene should fit the story. Simple as that.
To sum it all up...
Good sex scenes aren't just about the sex or what goes where. They should be integral to the story and the characters. They are defining moments in the book, and should be written with a level of care to reflect that.
Also, when I accepted a job as a teacher at a private catholic high school five years ago, I never would've imagined I'd be sitting down one day to write an informational post on writing good sex scenes. Go figure. Life is weird!