I've always thought it was a joke about if you go grabbing for a horse's or a bull's private parts you're going to get taught a lesson real quick about which kind of animal lets you go grabbing around down there and which one doesn't.
So anyone who says it's a masturbation joke ipso facto has a dirty, stinking, preverted, demented, depraved, sick, disgusting mind. They should be shunned and kept from the boy cheerdrens and the girl cheerdrens.
Like I told another poster.
My sister tried that when she was little with a Billy Goat.
She got knocked down.
And I agree about the dirty, stinking, perverted, etc minds.