I didn't even want to post this here because ...gosh...what an awful thing for the boy and the family to go through.
As far as learned behavior...I don't feel that way... I learned (in dog obedience school) that dogs view humans to be other dogs. In any group, there is the Alpha dog - the leader of the pack. As an owner, I was trained that I had to prove I was the Alpha dog in order to get mine to be obedient.... if this is true, the the 2 yr old playing on the floor could certainly be viewed to be "just another dog" to this pitbull. Dogs hump other dogs... I don't believe that we have a sexually deviate dog here...
And yes, proper training by the family is key...how many times have you gone to someone'* home only to be greeted by Rover humping your leg...most folks get a good laugh over it and push the dog aside. Unfortunately, a two year old cannot do such a thing.
This is a tragic situation that could have been prevented...it makes me sick to think about that poor little boy...but blame the dog? I don't. Last month a similar incident allegedly occured in Wisconsin with an 8yr old girl, according to one of the vets interviewed today.