My personal opinion is that men wanting to have sex with girls just turning into women is as old as the history of mankind. In that long history, it is only recent that this has even been deemed wrong (not terribly long ago, a pregnant 14 year-old woman was not considered odd or outrageous).
Our collective unconscious on the subject will take a long time to catch up with our current views. That's why it's a billion dollar industry - because it is standard fantasy fare.
I even believe that "normal" males can have this fantasy. When it's older "normal" males, I think it is often rooted in a desire for youth - to possess it and recapture some of your own. The lure of purity and innocence will always be a powerful aphrodesiac to humans possessing bodies that whither and die and minds that become jaded and world-worn.
These classic yearnings are present in all types of our art from Lolita to American Beauty. We can say "Eww - yucky - that's gross and sick" or we can explore the underlying realities in the face of the progressions of our own morals, culture and beliefs.
Yes, she is 15 and off-limits is today's society, but according to nature, she's a woman.