I posted this question because I realize my own view of the part pain plays in making a spanking erotic has changed over time. I am conflicted about my more recent view or, it might be more accurate to say about coming to terms with the emotion there all along.
Males spanking females or females spanking males, have been erotic to me for as long as I can remember, even going back to before I knew what arousal or sex were. In the early days of my spanking interest, I just figured I was a little weird to be aroused by spanking scenarios. It was only when I got old enough to encounter erotic spanking in literature that I found my kink was not even unusual.
What was erotic to me for many years was the D/s aspect, the removal of clothes, exposure of the bare bottom and view between the legs of the recipient, humiliation... I did not give pain a conscious thought. Only recently, watching a realistic spanking video, I realized the very real pain being suffered by the victim was arousing me. It was almost orgasmic when, completely overwhelmed by the pain, she gave in to uncontrollable crying. That response in me scared me.
Thinking about how coming to respond to the pain seems to have developed over a long period of time, I wonder if it was a case of more and more stimulus was required to trigger an arousal response. At first, just the lowering of panties and submissive positioning was enough but gradually that wore off and, finally, pain became the only stimulus to which I had not become anesthetized. But, for whatever reason, pain went from not even consciously registering to being erotic in itself.