Or how marriage began in India.

Following is one of the legends on the beginning of marriage (as in enforced lifelong monogamy) in India. This occurs in Mahabharata, Adi Parva, Sambhava parva as well as in other texts.

The legend goes, as follows:

There was a sage called Uddalaka. He had a son called Swetaketu.  Swetaketu was a small boy aged about eight  or 10 years.  Uddalaka was doing morning oblations(with water) to the Surya(the Sun).  His wife was supplying water to him.  While the oblations were continuing, another Brahmin came and made a proposition to the lady  that  they  should  go  together  for sexual intercourse  and  the lady readily agreed.  The boy was taken aback and he told his father Uddalaka:  "How is it that you have allowed my mother  to  go with another  man?"  The father, Uddalaka says:  "Boy, that is the rule of the land." Actually he said, "Be not angry. O son! This is the practice sanctioned by antiquity. The women of all orders in this world are free, O son; men in this matter, as regards their respective orders, act as kine" i.e. men are helpless as cows and must get cuckolded to preserve women's freedom.

But Swetaketu did not agree. He did not agree that men should get openly cuckolded for women's "freedom". What was the reason for Shwetaketu's anger? Actually, Shwetaketu was born at the same time as Uddalaka's grandson Ashtavakra. When Ashtavakra was still a young boy and playing on Uddalaka’s lap, in jealousy, Svetaketu pulled him down and stated angrily “He is not our father, he is my daddy.” Ashtavakra was shocked; Uddalaka had raised them both as his sons. Shvetaketu further told Ashtavakra that his father is dead. Angry at Svetaketu for revealing this, Uddalaka told Svetaketu that he was born to his wife by one of his disciples not Uddalaka himself. (Santi Parva, Chapter 35, Verse 22). Imagine your father revealing his own cuckoldry to you like a mean spirited child.

At a later stage, Shwetaketu became a great sage. He  wrote many texts, now available in multiple  Upanishads  and he also propounded the first theory of Uddalaka-Swetaketu principles wherein  adultery was prohibited. It has been observed that the first person who raised his voice against the evils of drinking; and adultery by the Brahmana community was Shvetaketu. He laid emphasis on monogamy and forbid polygamy also. He took his campaign to extreme limits and thus, Shvetaketu was expelled by Uddalaka from his house for his hatred towards Brahmins.

As can be seen from above, an adulterous woman was what prompted creation of monogamous marriage. Also it was initiated by a son, who wanted his parents to stay with him. However, when he drafted the laws, he didn't got prejudiced, and made the law gender neutral, i.e. he forbade adultery by both the husband and wife. Being a man, he was honorable. However, women do behave honorably.

(Note that Uddalaka was no mean scholar. His fame in India is exceeded only by his student Yajnavalkya and his son Shvetaketu. However, he was like a cuck professor - intelligent but helpless.)