One story could be that the husband has always been a bit of a prankster and once made a big mess while trying to surprise his wife with a DIY project. He felt so guilty that he told his wife he deserved a spanking as a punishment. His wife, in a loving and humorous way, gave him a light spanking. It was more of a playful act that strengthened their bond as they both ended up laughing about it.
There might be a story where the husband was feeling a bit down and unproductive. He told his wife that when he was a kid, a spanking from his mother sometimes motivated him to get things done. So, his wife, with a lot of love and understanding, gave him a very mild spanking. This simple act made the husband feel cared for and he started to regain his motivation and energy.