I am one of those for whom the spanking can be non-erotic at the time. After it is over all I want is to be held. I feel so completely vulnerable that what I need right after is comfort. I can not get enough body contact with my husband. He will often hold me in his arms on his lap until I feel settled again. It is a wonderfully warm and safe feeling. Is it still erotic? Sure it is but I am attracted to my husband as the authority figure. That is what the turn on is, not the smacking on the butt. It is the fact that he will take me over the edge to a place where I am completely emotionally vulnerable. He will hold there for a short time to allow his control and my vulnerability to sink in. Then he stops and holds me.
Like Diamond I will often relive the incident in my fantasies later, so it must surely be erotic. There is almost always sex after. My husband likes very much to make love after to a completely submissive woman. It is the only time I really am completely submissive. About the differences Sarah talked about. It is different for my husband. He finds the spanking part always erotic. He is turned on by the actual smacking part as well as the control part that is the big turn on for me. It can be different for different people in the relationship and still work very well.