You hit upon something very important there, Hom... I think we make a mistake in lumping all sexual offenders (against minors) together.
Firstly, there is the paraphilia of paedophilia, which simply means that a person does (for reasons we could speculate on all day, ranging from Pavlovian ideas of fetishes to having been abused themselves - I suspect it can be any number of things) have sexual urges involving children. They may not act on it, they may be ashamed. Or they may act on it and feel intense guilt as a result.
Secondly, there is the factor of either sadistic or narcissistic personality disorders, which not all paedophiles meet the criteria for, but some do. I think this could be of particular importance in cases of incest. Of a study that was done recently on people who had committed incest on their own children, only about 1/4 were actually more aroused by images of children than by images of adults. This could suggest that there is something else at work, something else they "enjoy" other than simply children. They may enjoy inflicting suffering, they may take pleasure in the fact that they are engaging in a sexual act with, essentially, a part of themselves. A kind of De Sade-esque joy in stepping outside the all moral boundaries of humanity.
Then there are the sad men who go for girls who are 14 or 15, post-pubescent, just not yet "legal". The attraction here is their innocence and lack of experience, and the majority of them are probably hopelessly inadequate bastards and, in the back of their mind, they know it.
My rather rambling point is just that it's a very complex area, and one which needs a whole lot more investigation. I have a cheek to say that though as I'd hate to be the one doing the investigating - the whole thing just makes my blood run cold