Actually typically it seems to be personal taste and the symbolism and sentimentality of it. Maybe that's why they seem to overlap with disliking ntr. At least this is my view of it.
Doubt the mother was raped as the comment of 'is your heart that weak' just seemed like a generic mocking from the villain to allow the author to give a reason for the debt. Never seemed like something for the story and more for the art and a reason for it being rough.