Question: Would you want your unaccompanied young daughter (8-12 years old) sitting next to a 37 year-old male stranger? I wouldn't anymore than I'd want her to sit next to any stranger and I am not around.
Of course, I am not stupid enough to send my kids on a plane by themselves. If they are on a plane, I'm right there with them.
Personally, I think that this whole thing was ridiculous. The man was right, what could he do trapped on a plane that nobody would catch him doing? If he did try to hurt that kid, the police would be at the airport waiting for him to get off. That is if someone on board didn't kick that shit out of him first.
However, from another perspective, as a parent, I want all children to be safe. Even though they were trapped on an airplane, some people are crazy enough to hurt a child even in that case. And, unfortunately, most pedophiles are males. So from that perspective, It wouldn't bother me if they had asked me to move to a new seat for this reason.
The reasonings for the airline doing this has more complexity that many people realize. There is no easy answer.