Is that so hard to understand? I mean really, you have both women giving their all 100% to you, and you're only able, at best to give 50% to each of them.
I had a Jewish guy who wanted me to be his second wife because his first wife couldn't have any more kids. I talked to his first wife and she said he wasn't paying the bills for her and her children. I asked him how he planned on supporting a second wife and kids, he said "I'll make the first wife get a job."
Geez these people really look at the "prophet's era" through rose-tinted glass... He was unfair, he threatened/spiritually blackmailed his wives, let his wives starve, etc. From the very beginning, there was no fairness where this one-sided arrangement would work for women.
Here's a thought experiment to demonstrate why this happens:
Let's say you're a security guard on the job. You are holding a taser. You see two men running towards you. The man closest to you is clearly trying to evade the person pursuing him, who is firing a gun at him. Which of the men do you fire the taser at (let's assume you're a good shot and both men are in range), the man who is clearly running for his life, or the man who is shooting at him? Now suppose that the man with the gun is a uniformed police officer.
Nothing changed between the two scenarios besides the clothing the person with the gun was wearing, and yet it completely reverses your actions, because you instinctively trust that the police officer, an authority figure, is shooting because it is necessary. If you apply a moment's thought to this, you realize it is not always the case and you adjust your answer, but your instinct tells you to trust the authority figure.
This problem doesn't necessarily mean they are bad people. Instead, it's a problem with how our brains are wired--we are wired to believe that our parents (and by extension, other authority figures) know what's best even if we don't understand why they're right. This makes sense--we don't have the survival skills other animals do when they're born. A newborn human can't find food, run from predators, or otherwise care for itself. So we had to take a shortcut and trust someone else to do the learning for us.
Here's a link to my whole long essay on it:
http://pathtokolinahr.blogspot.com/2015/01/moral-people-defending-immoral-things.html