My thoughts are developing, based on my own experiences and through reading and responding to others facing polygyny. Something came to my mind that is very important - the sacrifice that women and the children of polgyny have to make, and why.
When a man enters into polygyny, even when he is in the search process, the existing wife has to give up some of her time with him. He may spend hours searching, then "communicating," then preparing, and then dividing up the time between the existing family and the new wife. The children also have to give up some time, some involvement. There is no comparison between the time invested in the children in one household, and the time invested between two, especially as there will be more children to divide time between. He will also be dividing his money between two households. He took your pie and cut it in half, or thirds, or even quarters, giving the other half to another woman/women.
Many beautiful sisters out there have the right mindset and intentions and accept polygyny, desiring their husbands happiness, to successfully implement a Sunnah, and the pleasure of Allah. However, there will be times when they will stare at it and see a stark reality that they are sacrificing a lot for the sake of their husband and his desires. At times it will appear, with the help of shaytaan, that the husband is getting a really good deal because he gets what he wants but the existing wife is left with loss, pain, insecurity, and shaytaan happily and firmly attached to her ear.
Often we wives sacrifice to safeguard the husband's Islam, to prevent him from haraam, because he is driven to pursue his desires or cannot be satisfied with what he has. When I look at it from a certain light, it seems that we women who sacrifice so much to support their husbands in polygyny, out of love, are much stronger. After all, we don't turn their lives upside down chasing after our desires and finding solutions to our weaknesses. We sacrifice because we look at the greater goal of Jennah; we see it as yet another test. For the polygyny chasing men, they get caught up with the dunya, by way of their desires and urges for the zeena of this life - women, children, wealth, etc. - and they must satisfy much of that desire to be able to get past it and focus on Jennah.
So where does that leave us wives? I believe that polgyny is sent to very special women; those that Allah wants to test strongly in order to increase their emaan, taqwa, tawakkal, and ibadah and purify their hearts. I believe that it is sent as a means to raise their ranks in Jennah and as an opportunity to be amongst the muqarraboon, the forerunners closest to Allah, in the aakhirah. We know that Allah tests those He loves, and for such a great test, there must be great love.
Jâbir b. `Abd Allah al-Ansârî relates that he heard Allah's Messenger (peace be upon him) said three days before his death: "None of you should ever die except while assuming the best about Allah." [Sahîh Muslim (5125)]
Ibn al-Qayyim says: There can be no doubt that assuming the best about Allah only comes as a result of righteous conduct. It is the righteous person who assumes the best about his Lord, since he knows that Allah will reward him for his good deeds and will never break His promise.I want to be with the muqarraboon, and I begged Allah many years ago to make me an excellent person, so I take the tests and know with certainty that Allah is sending them to make me even more beloved to Him. No question, the tests are very difficult at times, but greatly blessed!