The thing is, it’s difficult to treat all the wives equally. Even Islamically, if I’m not mistaken, the Prophet (saw) had a difficult time because of jealousy among the wives and that was when Allah (swt) sent the ayah that marry up to 4 women, but only if you can be just with all, otherwise keep one wife.
The second wife will have children… she’ll be the fertile one, getting more attention from others and maybe even from the husband. It’s easy for the first wife to be pushed aside. And if the wife cannot have children, I think it’s better to adopt and get sawab.
Sara nailed the reason why polygamy is difficult to accept. I don’t think it’s really because women are becoming more financially independent or there are fathers and brothers to protect her. A financially secure woman and a poor woman can both be possessive of their husband. It’s human nature… no one wants to share their spouse. A husband wouldn’t want to share his wife and vice versa.