Here's my theory of what the problem is with writing sex scenes. (Oh, and I'm willing to be wrong about this, so don't hesitate to disagree with me. All alternative theories will be considered seriously.)
But before I get to the theory, I want to make clear I'm not particularly hung up on the "classification" of sensuality in my writing. I figure I'll write the book and that will tell me how "warm" it is. Clearly, if they're having sex, though, it's not one of the "chaste" sub-genres.
Okay, so my experience as a reader is that sex scenes in most romances "feel different" from the rest of the book. The best way I could explain it was to speculate that the author goes from being very lyrical and expressive to writing very explicitly about anatomy. It can be jarring. I'm not the only reader who's said -- and no, I don't think this is a polite lie or self-delusion -- that I skip sex scenes. Or read one (so as to be assured that the characters are sexually compatible) and skip the rest in that book.
As a writer though, I've really thought about this phenomenon. I'm working on a contemporary novel, so I want to find the right balance between realism and romance-novel idealism. This means I don't want my hero to attack the heroine -- it may seem sexy because it means her sexual appeal is so strong that he can't control his desire for her, but in real life it would suggest that he's (as my husband put it) "not a nice guy." On the other hand, I want my hero & heroine to have "smooth" sex -- meaning her hair doesn't get trapped under his hand (ouch!) or the phone rings at the wrong time. That would be as distracting to read about as it is to experience.
I'm very committed to making good use of point of view: if I'm in my hero's head, I want to limit myself to those thoughts, feelings, observations and experiences that he would naturally have. Most of the time that's no limit at all; my hero's a bright guy. But in the bedroom? What do men really think about?
I asked my husband (pretty much the only guy I'm entitled to ask, if you think about it), "Honey, what do you think about during sex?" He's a bright guy, too, but he didn't shock me much when he answered sardonically, "Not a lot." Because of the four things we get from the hero's POV (thoughts, feelings, observations and experiences), thinking falls away dramatically as the moves and rituals of sex progress, feelings get very focused on a relatively small number of nerve endings, observations become more intense but monosyllabic (e.g., "that feels good,"), and experiences -- well, there's the rub. (Please forgive all inadvertent but predictable double entendres in this post.)
Obviously, my hero's experiences are as lush and wonderful as we could wish them. I can write about them too, but I risk pushing myself out of his head if I do so. He's not cataloging his experiences as he has them, he's just having them. And when in real life thinking, feeling & observations all reduce to a Dr. Seussian economy of words, why would it be appropriate for the text, in the hero's POV, to start noting all the places that body parts are going and all the things they're doing?
Go read a generic sex scene in a generic romance novel (but not a paranormal -- who knows what powers the 7-foot-tall hawt vamps have). Ask yourself, Is it likely that a real human being in this situation would be aware of all the things I'm reading about? I get it that those things are happening, but would the character be aware of all that? Particularly as -- how do I put this? -- the experiences reach their intended conclusion.
Now, I'm not suggesting that sex scenes in romance novels (and here I'll explicitly exclude erotica from this discussion) should devolve into grunts of satisfaction or cries of passion. That would be boring to read. But why can't a sex scene be more realistic about the way a person's focus narrows and his/her cognitive sophistication shuts down?
I think what happens is that a lot of authors pan back in those scenes and start writing as an omniscient narrator while still claiming to be presenting one character's POV. It just doesn't seem to be necessary. There are a limited number of positions and acts that two protagonists engaging in consensual vanilla sex can get into. Let the reader use her imagination -- if you really need the play-by-play commentary, have a character replay the scene in his or her mind after the fact. Then you can have every last square inch of skin mapped out and written about in detail. They may not be thinking a lot while they're doing it, but they can remember it all.
I've tested this theory this in my current WIP. I think it went okay, although I know I'll need to add more depth to the scene in subsequent revisions. I'll be the first one to admit, though, that no one will be able to get tips and pointers about the latest sexual techniques. For that, you still need to read Penthouse Forum.