I think it works along this way:
In rural Pakistan and India, the houses are small. There is often only one bed to sleep on, so the whole family sleeps on it. This is not an uncommon practice - it was once common to see in England as late as the Victorian era. You would see it in North America until the Edwardian Era.
The local justice system is not very robust. Because family beds are so common, so is incest. The only way to keep incest from happening is demanding a virginity test. When a newlywed couple hames for the first time, the sight of blood is watched for. (Or at least, that was the ancient English tradition.) Anyways, if the test is failed, the daughter's father is shamed and faces serious sanctions. He might not be able to marry off his daughters and his family goes down a few notches in the heirarchy. This can affect stability a great deal. It can mean the family loses land or lose business; they might not be able to get food as regularly.
So that is the risk.
Because it is so competitive, they develop a labyrinthine, byzantine and ultimately self-contradicting system of traditions and beliefs that it leads to such silly conclusions.
But now they have to face a robust justice system and that is going to suck for the parents.
