The man could also be very supportive. He may join in the conversation later to reinforce the message his wife was giving. Say the wife was teaching the son about sharing, the man could later share his own experiences about sharing when he was a kid to further drive the point home.
The husband could also step in after the spanking to comfort the son while still reinforcing the lesson. He might say something like 'Your mother spanked you because what you did was wrong, but we still love you.' This helps the child understand the discipline was about the behavior, not lack of love.
One story could be that the wife calmly sits the son down and has a serious talk with him about his misbehavior. The man watches as she uses words to make the son understand the consequences of his actions. For example, if the son broke a neighbor's window while playing ball, the wife explains how it made the neighbor feel and that he needs to save up his allowance to contribute to fixing it.
In some stories, the husband may be conflicted. On one hand, he doesn't want to undermine his wife's authority, but on the other hand, he may not fully agree with spanking. So he might wait until after the incident and have a calm discussion with his wife about their parenting styles and how they can be more consistent and positive in disciplining their son.
He could have different reactions depending on their family's discipline philosophy. If they generally agree on strict discipline, he may support his wife's action silently. However, if he believes in more gentle ways of teaching, he might step in later to comfort the son and have a talk with his wife about alternative methods.
He could be in shock. Just like finding out something completely unexpected that turns his world upside down.
One possible reaction from the husband could be silent approval. He might believe that the son has done something seriously wrong and that the spanking is a necessary form of discipline. For example, if the son has repeatedly disobeyed a safety rule, like running into the street without looking. Another reaction could be discomfort. He may not fully agree with the physical form of punishment but doesn't want to interfere immediately in front of the son, so he just stands there uneasily. Or he could be empathetic towards his son. He might try to comfort the son later in private and also have a conversation with his wife about alternative forms of discipline.
She could be curious. Just seeing her husband stroking something might make her wonder what he's thinking or feeling at that moment.
Glee might be excited. Fanfiction often has some really cool and unexpected twists that can make for an exciting read or viewing experience. They could be really into the new takes on the Flash's powers or the relationships between the characters.
Some of the Hetalia characters might be offended. If the Hamilton fanfiction distorts or misrepresents certain historical events or figures in a way that goes against their understanding or values related to history, they could be put off. For instance, if a fanfiction portrays a historical event in a way that seems disrespectful to their 'country', they would likely not be happy about it.
One possible reaction is confusion. She may not understand why her husband is doing this in a situation where she can see. It could also lead to an argument if she feels it's inappropriate behavior in a shared space. Or, if she has a more liberal mindset, she might just casually ask him about it and start a conversation about personal boundaries.