How does polygamy benefit the state? It's a serious question.
With regular two-people marriage, you're roughly giving almost everyone a shot at finding a partner for marriage. This means you don't get large roaming populations of one sex or the other without any hope of finding a partner, and all the problems that can bring (see China). Those people will not be able to participate in the state-approved stability that marriage can bring.
Gay marriage is actually positive for the state in these respects, since it expands the number of people who can participate in the current framework. Polygamy is much more complicated at scale, and I think you'd need to find solutions to many other potential problems before you could convince the state to accept it.
With regular two-people marriage, you're roughly giving almost everyone a shot at finding a partner for marriage. This means you don't get large roaming populations of one sex or the other without any hope of finding a partner, and all the problems that can bring (see China). Those people will not be able to participate in the state-approved stability that marriage can bring.
Gay marriage is actually positive for the state in these respects, since it expands the number of people who can participate in the current framework. Polygamy is much more complicated at scale, and I think you'd need to find solutions to many other potential problems before you could convince the state to accept it.