I just touched on this in the
tranny centaur thread.
Psychologically, male fans of big dick porn are alleged to have homoerotic tendencies, but the most accepted reasoning involves the subconscious attraction to the concept of a woman being vulnerable to a dick that may seem too big for her (virgin female ideology).
When she is able to handle (and overcome) the big dick in question that also feeds the fantasy of her being an insatiable, sex-hungry whore (I mean that in a good way).