Hinduism & Sexuality: PolyAmory in Ancient India
There was a sage named Uddhalaka who lived in the Ancient India , he had a son named Shvetaketu, Shvetaketu was a great ascetic himself. Upanishads have records of the teachings of these sages.
Uddhalaka was sick and bed ridden. A man entered the house and caught hold of Shvetaketu's mother's hand and said "Let's Go", they went outside (to a park) to romance. Shvetaketu who was seeing all this was enraged. How could she abandon her own husband in this position to romance with another man.
Wise Uddhalaka said, "Oh son, don't be angry, this is a legitimate practice, that has been practiced from the ancient times, Women have their freedom to do as they wish."
The Hindu classification of Ages into SatyaYuga, TretaYuga, DwaparaYuga, KaliYuga has it's importance. Stories from SatyaYuga depict a very free society, sex outside marriage wasn't considered taboo. Having a child was however restricted within marriage for the sake of keeping the family tree.
When people have higher comprehension, less chance to make mess out of their freedoms it is natural that they have more freedom. Those who sought romance outside marriage were still great sages, men and women alike. For those who spiritually awake sex was just another thing to enjoy, they had voluntary control over it and did it consciously rather than compulsively. In deed it is the best way to live regardless of the age on lives in.
As the consciousness decreased, people had to be stopped from harming themselves with the freedom, societies became rigid.
Note: Story is from Sambhava Parva or Mahabharata: Adi Parva: Sambhava Parva: Section CXXII; Other versions add more detail.