Muhammed did not have pedophilia, I think most people here understand that, and use the term pedophile are using it more a way of pointing out his perverted actions and how his crime would be labled, not as a clinical diagnosis.
That is important, so lets get that out the way.
He was however suffering from narcissistic personality disorder. He probably wanted her mostly because he could, because it was something different, because it made him feel powerful.
And yes, it would be taboo. This 'well that is how things were stuff' is nonsense. It wasnt. In ancient Rome women typically did not get married till their late teens or even 20s. Even in cultures where very young marriage was common, the two would not really 'be married' and consumate that marriage at 9.
Furthermore, even in cultures where child marriage happened, it was between 2 youngsters, an age difference of over 40 years? A 50 year old man with a 6-9 year old girl....That would be taboo.
And even if it was common in pre islamic arabia (it wasnt), so? He's supposed to be a prophet right? Perfect man forever. He took on every major institution in arabia, the whole system....oh, but he didnt want to address the raping of children or slavery...because ya know, that was just how people are back then...?