The modern world has evolved a series of rules on what is acceptable behavior, but this has not necessarily stopped abusive relationships. Abusers have learned that being physical with their partners may cost them the relationship, so many of them have found other ways to carry out their goals. They might ask for permission, but they disguise it to get agreement from their partner. Once they get permission, they then remind their partner they agreed to the abuse. This is a way to abuse a partner as well as control them, and it is an important red flag to notice.
Controlling a partner physically has long been a way to be dominant in a relationship, and abusers work hard to remain in control. Getting permission to do this is actually quite easy, and many of them have found it keeps them from being held responsible. They often seek control by posing a hypothetical question that sounds reasonable, but their partner may be unaware they are giving permission for abuse.
When an abuser is asking the hypothetical question, they may phrase it as a general inquiry. They believe doing this allows them to step away from responsibility if they receive a negative answer. Setting up a question by remarking that sometimes people are pushed to the limit and need to hit a partner is often how this scenario works, and then they ask their partner how they feel about it. If their partner agrees that there may be some circumstances where it is acceptable, the abuser takes it as permission.
The issue with dismissing this as a red flag is that the abuser feels they have been granted permission, and they will repeat the conversation back to their victim when abuse occurs. Even if physical violence only happens once in this type of situation, it is time to leave because even posing the question points to a person who believes their behavior is acceptable.