I admit, I disliked that change, but it wasn't just the "but that's Jamie's thing!" aspect of it, it was the change to Frank's character in general. As Frank is presented in the first handful of chapters, he seems somewhat selfish, and oblivious to Claire's needs. They go on a few day trips together, but often Frank leaves Claire to her own devices while he goes off to do his historical research and even when Claire is invited along, she feels ostracized (the tea scene goes very differently in the book.) Also, at this point, I think they're actively trying to start a family, which is another strain on their relationship, and I got the impression that although they enjoyed one another, the sex wasn't primarily for pleasure, so oral didn't make sense to me.
I do agree though, given how confident and in tune Claire is with her sexuality, it's silly to think that she'd never been gone down on. I mean she did it for Jamie... with teeth, so it's not like the concept was completely foreign to her.
He did use a horse analogy as his inspiration.