It is about how bigamy is unacceptable on this show.
Obviously nobody is a fool here. Nobody protested against Akbar's harem or Dasarath having three or more wives. That was the norm of that period and it was part of history.
In this show, Rana Indravadhan Singh has already declared that he does not believe in bigamy or polygamy. He was not even willing to accept living under the same roof as fake Sulakshana, who he thought was his first wife returned from the dead. He wanted to remain a husband to Gayatri and Gayatri alone.
To justify bigamy on this show on the grounds of period is a big joke under the circumstances.
It does not matter whether it was legal or not!
What matters is that this is the love story of Rana Indravdhan SIngh and Gayatri Devi and that is how it should remain. Bringing co-wives dilutes the very core of this story and hence is not acceptable.