There are times when any reasonable person would think spanking is okay. Particularly if the consequences of what the child is doing could be signficanly worse than the spanking. Like for an example if your child runs into the street without looking for traffic, or if your child is about to touch a hot stove. Being spanked is clearly better than getting hit by a car or being burned.
However many parents spank because they are angry at their child. They spank to make themselfs feel better. You should NEVER spank out of anger. If you do that is abuse.
Also IMO physiological abuse is far worse than physical. Children get hurt all time through play or what ever, but they recover very quickly. However when they are physically abused, it is the physiological componet that makes it so bad. Also we need to have just as much outrage when children are verbally abused. Those scars last a life time.