Such an explicit level of consent is not common in vanilla sex. Because it is assumed everyone knows what is expected sometimes assumed where it does not exist. In several cases the basis for this belief amounted to little more than coming back to his room and kissing. Perhaps in the past this might well have been a reasonable assumption -it is not today. The level of consent that the law currently accepts seems to me totally inadequate.
Consent has to be explicit in SM sex because there are no fixed conventions.
But ultiately that is true of us all. We live in a multi-value world where conventions change rapidly. The more liberal concept of consent is not suited to today's more fluid society. I would add that consent can be the only exception and hence I think it is a pity you do not question the legality of lawful chastisement. From an SM perspective the canning of children can only be regarded as rape.