I think that in this case I'll go with the book over the movie. I'm currently writing a fan fiction, though I wouldn't say it's sick, there is a chemistry in the story between these two characters, though it is very subtle. Lucy is closing in on 19 in this story, so there's nothing sick about it. In real life, we read in the paper about celebrities who are in their 50's dating women who are in their 20's, and if we can get beyond judging them, them perhaps there is hope left for fan fiction as well.
Yes, I will admit that some of the fan fiction is far out there, some I will not even read, as a lot of it has absolutely no storyline, no plot, just this pairing stuff, and generally poor grammar and spelling. However, in defense of those who do try to write a good story or weave a good yarn, it's not really fair to lump all stories that link these characters into one batch as being 'sick'. I have seen quite a few stories with Lucy and Tumnus that are exceptionally well written, the stories themselves are believable and there are good plot structures and themes in them.
Having read some of the comments here on this particular topic, I don't really know if I will post my story when it's finished. I am not afraid of constructive critiques, however, I do not like to post where only one type of story is acceptable and the rest are shoved under the rug. I have written stories since my early twenties, and I like to read whatever it is I read with a fair amount of objectivity.
The other story I started posting here, no one seems interested in, so I may just post another part or forget about it completely. Although, this is not a pairing story, but it was written several years back, and it is a story where a girl ends up in Narnia, but there is no romance in it, as the girl is 8-years-old. The new story, I don't know if or when I will post it. I have really been dragging my feet for years about registering at fanfiction.net, but that seems to be the only place where Narnia stories are actively posted, and yes, I have been known to read a few of them.