It could also be about acceptance. The girlfriend watching could imply that she accepts all aspects of her partner, even the parts that are vulnerable or in need of correction. Maybe it shows that in a relationship, we should be there for our partners through both good and bad times, and not shy away from seeing them in difficult situations.
One possible moral could be about responsibility. If the spanking is a form of punishment for bad behavior, it shows that actions have consequences even in a relationship. For instance, if he had been irresponsible with money and this led to some sort of 'spanking' situation, it teaches that one should be more responsible in a relationship.