@shruthi Gayetri did not go after a married man. If she made one mistake (and a lot of woman do that) is to stay in spite of not wanting to be second wife.
And it was 1940, British era. And as per British inheritance law of that era, only one wife got the right. If the first one was the wife, second one is the mistress. It was not pre-british era when multiple wives were staple. Why do you think BRM brought back Sulakshana look alike at that particular time?
Because A. Rana, having a soft spot for Gayetri, won't try to have heirs with fake queen (I am pretty sure Jhumki also got the warning to stay sick so Rana stays away). And B. Having a first wife, second wife's children will be in the lesser category as BRM's children for the line of throne since they are legitimate and the second wife's children are not considered legitimate.
That's precisely why BRM waited until Rana had feelings before bringing back Fake queen. Otherwise, she would have done it when Rana hated Gayetri. He would have happily kicked her out of his life then if that's all she wanted. But that won't have put her children in line of throne. Only having Rana away from First wife yet still considering first wife as queen would have made it possible for her son get the throne as per British law.
And by crowning Sulakshana, Rana played into her hand.