@Ionus,
Marriage is a contract between 2 people. It grants rights under the law to the 2 people in that contract. To restrict it to only certain types of 2 people like whites, heterosexuals, the rich would be to deny equal protection.
It doesn't deny rights to poly relationships since marriage law is about the rights one person has when married to one other person. When you introduce a third person then all those rights are no longer available simply because under marriage law the rights are granted to 2 people equally and result in one person having rights when the spouse dies or is incapacitated.
In a marriage, the spouse inherits all without tax consequences. In a polygamous relationship, no single person can inherit it all so tax law would trump the poly marriage making the benefit not applicable.
In a marriage one spouse can make legal and medical decisions for the other if they are incapacitated. In a poly relationship there would be no one person to make those decisions so it wouldn't fall under current law.
The law is filled with many benefits for marriage that would not be available to a poly relationship because it is not 2 people.