I did know. I had written four chick lit novels where all boinking happened off the page. There was some warm-up activities, some heavy breathing and then my characters would wink and shut the door. We all knew what was happening, but I didn't describe it. In a romantic suspense novel, my first to be shelved in the actual romance section, there would have to be on-the-page boinking. I felt like I totally went for it. I specified body parts and which parts were touching the other parts and how the parts felt and everything.
Romantic Times rated the book "mild." Mild? MILD? They had no idea how much teasing I endured from friends and acquaintances about those sex scenes and how exposed I felt when writing them. I had some great reviews for that book, but not a single one of them said the book was sexy.
Then I wrote Don't Kill the Messenger. Again, my agent brought up the boinking. I said I had it under control. Once again, it's on-the-page boinking, but since this was more an urban fantasy I did dodge a little bit. There's not as much detail. I'm pretty sure everybody knows what's happening, but it's slightly more euphemistic. The scenes have a lot to do with my heroine relinquishing control a little bit and the give and take between these two strong people.
Guess what? Almost every review has had the word "sexy" in it.
So what have I learned? The sexy is not in the details. The sexy is in the mood and the interplay between the characters. Even if there are other body parts on the page, the sexy is still all in the head.