Before the cherem of Rabbeinu Gershom when it was permitted to marry two wives:
1) Did the husband need the permission of his first wife to marry a second wife?
2) Is there a mekor that even before the cherem it was considered shameful to take a second wife unless the first wife was barren?
Essentially it is permitted if he can afford it and provide each of his wives with their own home. However if the area that they live in does not permit it, or if it is written in the kesuba that he will not marry someone else, then it was not permitted. However if the minhag in that place is to marry two women, and the woman didn’t specify that she doesn’t want it then he ios allowed to. It is difficult for us in the 21st century to understand the mindset of the people who lived a thousand years ago, and it is also difficult for us to understand the mindset of the men and women, and their relationship regarding marriage. On one hand a co wife is called a tzara- an affliction, because it is very difficult for a woman to have competition for her husband, but on the other hand we don’t really understand their mindset
Kiddushin 50b, Even Haezer 41-1, 76-7,8.