I don't think it was a straw man at all. I have seen people take exactly the stance that I attributed to you as an overreaction to other people saying that spanking is not abuse. There was no intentional misrepresentation--GP said "Spanking is child abuse?" and you replied "Inflicting physical pain is child abuse." That rationally includes all physical pain regardless of intent. It turns out that what you meant is rather different than what you wrote, so thank you for clarifying.
I don't believe backtalk is in line with medical treatment, though I failed to make that explicit. There was another poster who mentioned that he spanked his son only twice, and one or both of those times was when he found his son doing something incredibly dangerous. The example that's stuck in my head is finding a child playing with a gun--and I recognize it's a poor one because responsible parents keep them where kids can't get to them. But in an instance like that, where the child cannot be expected to understand why he can't do this particular thing, it may take more than stern words to stop him from repeating it. And that's probably the approach I would take--first, get the child out of harm's way; second, tell him sternly never to do that again, and why it is dangerous in case he can comprehend it; and third, remain on the lookout and if he's heading towards the danger again, then maybe it's time for a spanking. I believe that for it to be effective it must be used sparingly, and for it to be punishment instead of violence it must not be done in anger.
I do not have children, so this is all hypothetical. I agree that there are problems in reasoning, implementation and severity. That says to me not that the entire practice is barbaric, but that people can misuse and abuse it. The same can be done with many (any?) form of discipline. Do you see a difference between inflicting physical pain versus emotional or psychological pain as punishment? Doesn't most punishment involve some kind of pain, even if it is the pain of having to sit on the couch while your siblings continue to play? That sounds trivial to an adult, but it is far from it for a child.
I had a good conversation with someone here a year or so ago where we discussed this pretty thoroughly, and I came away with more respect for his point of view than I had before. My own reaction against those who choose to never spank their kids is that that often means they do not discipline their kids at all--you can see examples of this all over in public: kids being given whatever they want in order to keep them from causing a scene--t's like the kids hold all the power and the parents none. So my perspective was almost the complete opposite of his and I suspect yours. Discussing the subject in-depth helped me to see more ways in which alternatives to corporal punishment can be applied, and I think my conversational partner left feeling like maybe it isn't the great evil he thought it was coming in. I probably would have responded to your post even with the "as punishment" qualification, because I think that's still overly broad and I am genuinely curious about the aspect of emotional and psychological pain. The tone of my response would have been more like this post than my previous one, though.