The thing about sex scenes is they usually don't communicate anything that couldn't have been communicated just as well or better in more subtle ways.
That's not always the case. Sex scenes do make sense sometimes. Ironically, the first example that comes to mind is clothed sex scene in the movie Hot Shots, where Topper Harley cooks breakfast on the woman's sizzling belly. I know that's not a good example.
The sex scenes in A History of Violence are a better example. The story is about a reformed man who is forced to face his thuggish past, and forced to realize that those thuggish inclinations remain a part of him. Both sides of his personality are shown in two very different sex scenes. In the first, he is gentle and giving. In the second, he is brutal and selfish. Since the story as a whole is graphic and adult-oriented, using sex scenes to show this dichotomy makes sense.
On the other hand, consider the movie Big. That film shows Susan unbuttoning her blouse and John putting his hands on tits for the first time, but then camera fades out before even her bra is off. It's a simple, subtle scene that shows little and reveals much. Later, the characters laugh after Susan expresses some anxiety over the relationship. John leaps onto her and the camera cuts away. Sex is again implied, and all that the storytellers wanted to convey is expressed without actually showing the sex.
To Catch a Thief with Cary Grant and Grace Kelly is another good example of subtlety.
Mystery can be a powerful tool. In horror stories, keeping the monster/ghost/psychopath hidden or mysterious often increases suspense. It allows the audience to interject their own fears and suspicions, thereby personalizing the story... as well as demanding much of the audience's concentration and making them more susceptible to awe and suggestion. Likewise, mystery surrounding sex allows the audience to imagine what they will, allowing for personal expectations. It can also preclude conscious consideration by moving on quickly, thereby encouraging the audience to react with gut instincts.
Not showing sex doesn't reduce its presence or impact on a story. Dracula is a great example. The vampire's habit of biting on the neck is suggestive and provocative. Ever since Stoker's novel, vampires have been associated with sensuality. Stoker wrote more graphic scenes, as have many authors of vampire stories, but it is the neck-biting that most audiences remember.
The increasingly common inclusion of sex scenes and their increasingly graphic nature is more a cultural trend than an artistic one. I've seen almost as many films made before the Sexual Revolution as after, which is rare for someone of my generation. And I'm sure many of you are familiar with many classic works of literature, like Oedipus Rex or The Taming of the Shrew. These stories don't shy from sexual themes, but instead present them in thoughtful, rather than obvious, ways.
As is often the case in art, restraint can inspire. Just think of your daily conversations. Who usually speaks more artfully -- the person trying to be polite and considerate, or the person who accepts no bounds? I cuss quite a bit on occasion, but it's when I'm not cussing that I speak more eloquently. Try to tell a dirty joke in a clean way; you might find that the nuance makes the joke funnier. Restraints are challenges, and challenges feed the imagination.
As I said before, showing sex explicitly certainly makes sense sometimes. But I think trying to present the same themes without obvious presentations often leads to better results.
The same goes for making particular characters sexy -- try to make him or her sexy without any brazen comments, without tight clothes and a lot of bare skin, and you might be surprised at the effect.