I'm listening to the audio version of The Gentleman and the Rogue and, wow, there is a lot of sex in that book. I think most of it is in the first few chapters (I'm checking the first half, Bonnie gets the second).
There is a "don't listen to this if you're a kid" disclaimer--and added to that is a "don't try anything in this book" disclaimer. Heh.
I don't think we have another book with that much boinking, frigging or sucking in it. Maybe that's why it's our most popular story?