There is no other way to look at it: spanking is a form of child abuse. Parents should not hit their children, and spanking is a lazy way of instilling discipline. The best way to guide children is to show them the consequences of their actions and teach them right from wrong.
During infancy, children cannot be reasoned with. There is no logical punishment that they understand. They still need rules and enforcement mechanisms. The enforcement mechanism that has worked for as long as there has been children has been pain. Pain can be a useful tool. Do that, and it hurts. Even the most primitive of impulses will understand not to do that again. However, pain is not always the answer, nor would it do any good if not immediate and related to the activity. Inflicting pain that leads to injury is also inappropriate. This is the grey in the conversation.
In some cases, spanking can be a very effective method of negative reinforcement. In other cases, it can be physically or emotionally damaging (especially if done too violently). It's important to know your child and recognize the type(s) of punishment that would be the most appropriate in a given situation.
No, I do not consider spanking a straightforward black and white issue, because there are good things and there are bad things about spanking. In the Bible, it says to spare the rod and spoil the child, but psychologists say that spanking can be very confusing for a child. There is not a clear answer.
Issues are rarely black and white, so I believe there is a gray area for spanking as well. I have occasionally used spanking myself and as a rare punishment I find it to be helpful. I certainly wouldn't want to support spanking that leaves marks or somehow physically harms a child for more than a few seconds.