Why would a man take on multiple mothers in law?

Because his mothers in law are not common mothers in law. In polygamous families mothers in law are subjected to the rules of whatever their husbands say, and are used like cattle, so if you were a man in such a situation, your mother in law would not be allowed to stick her nose into your business, and you would not be having to take her on shopping sprees because she would be making her own clothes and things.
 
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