First of all, let me thank you for keeping an open mind about this topic.
Equal marriage is one such area where woman in certain religion (would not name it to avoid ban) has an inferior right.
In most polygamous households, the man is the primary earning member and his wives are dependent on him financially. Do you think the first wife has the courage in many cases to deny the husband his wish for a second wife when she is financially dependent on his husband. She might fear that saying no might provoke his husband to divorce her leaving her without any financial and soceital support.
This is where a govt law banning polygamy would have forced the husband to not raise such a question in the first place.
You raised a question from 2nd wife's perspective, whereas I am more worried of the rights of the 1st wife and her independent decision making to say NO to a 2nd wife.
In the very specific situation you described, it is possible that the wife may feel intimidated to ask for divorce. That might be true in a society where women are not given equal job opportunities. However, that is just not the case these days. And if there are countries where women are not given equal opportunities, then that society needs reforms.
India is infringing on the rights of muslims to practice their faith freely. Muslims should be allowed to live by their own laws, and Hindus by their own.
Islam came and put a restriction on the number of wives one man could have. No more than 4. They have to be given equal time, love and assets. If the man can't do that kind of justice, than its best that he doesn't go for more than one. And of course, keeping in mind, that the 2nd wife has to decide to marry an already married guy. No body is forcing her. And Islam also gave the right of divorce to the first wife. There are plenty of men out there. No body is forcing her into a polygamous situation. She is free to leave and find someone else.
I am not seriously advocating polyandry. I merely raised this point to highlight that if polyandry is supposedly such a taboo than why does society accept polygamy so readily. In a modern society, both should be seen in the same context.
In medieval times, there were lot of wars leading to a lot of widows and orphans. Then polygamy may have made sense. But it does not make sense in modern setting.
You are right it is seen as a taboo, because that's how Humans have developed. People are free to choose here in the west, yet guys consistently choose not to become the second boyfriend. Similarly women continue to choose to marry an already married guy. And they have been for time before written history.
Similarly if there is a woman who thinks that she wants two husbands. She is free to leave Islam, and choose a different religion, or no religion at all. Whats the fuss? Why are you trying to force Islam to change, if you don't like it you don't have to practice it. Nobody's shoving it down the throat. You are free to choose your path in life. Its funny, you are like, hey I don't like this thing in Islam, hey that's your opinion, don't be a muslim. Simple.
Summary:
1- Islam didn't raise the number to four, it lowered and restricted it to four at a time in history where there was no limit.
2- Islam is the first give a right of divorce to woman
3- Its a right, no body is forcing anyone to stay in that situation.
4- 2nd wife chooses it, and so does the 1st
5- Its the woman's right if she decides to stay or not
6- Its the mans right if he wants somebody else or another
7- Man doesn't get a free for all, there are rules and laws, and its difficult to do justice to all, and he may not be able to have a 2nd wife if the first one leaves him, that way he only gets to have 1
8- Most Muslim women have no problem with this
Also important to remember that Islam has many rights just for women. For example, if a woman is earning her own, she is not required to share that wealth with her husband. Where as it is the mans responsibility that he must provide for her. Did you know that?
Islam is so well balanced out, in fact most western societies are perhaps more Islamic than some muslim countries. I would advise you to study Islam deeply, see the whole religion. Then you can compare it to all the other religions, and you can be the judge your self. Don't stick with the famous Islamophobic tropes, that really have no ground what so ever.
As for this issue, it is simply nature. How Humans are, women are ok with being in a group of concubines where as men don't.
You should really be asking the question to 2nd wife, why she doesn't have a problem?