I believe Nature eroticized spanking so that we would be willing to receive the pain that serves as a teaching tool for our own good.
As a Disciplinarian, I've often heard stories from people who, when they were young, spied on a playmate being spanked - hard - over the knee and on their bare bottoms. They cringed at the blows the mother leveled on his sore behind, and yet they were fascinated. The reason being; although he was too young to process this, it was, to him, erotic.
It is, after all, the bare bottom.
And from a very early age we feel our bottoms are private and a vague sexuality, begins. So it's not uncommon to find later in life, whether he was ever spanked, or not, that he needs to reconnect with the experience. And in my opinion, he's one of the lucky ones.
He benefits from being able to lay across a strong woman's knee to receive, the sound and painful, spanking which sends a message directly to his bare bottom, straight to his brain from her bare hand; making sure it all 'sinks in'.
If it's punishment that's required to rid him of guilt or make him tow the line; or if he must cry in order to cleans himself inside, he is now under the same care and good stead as a child of a strict and concerned mother.
So again, I believe that Nature eroticized the pain of spanking discipline so that we would learn to love it.