The custom of Live-in relationship is very old in India, wherein zamindars and Nawabs not only had many wives but quite a few live in-women. In those times it was not considered immoral to have a live-in relationship.
Post Independance, monogamous marriages became the in-thing. India still remains a conservative society where the institution of marriage is considered sacred. Plus chastity on the part of a woman is highly rated. The woman with a history of pre marital relations with a man will have less chances of getting married in the future.