Marriage in the eyes of the state is a legal contract between consenting adults. So, it shouldn't matter the gender or even the number of individuals in the contract. All that matters is that they are of some form of sound mind, consenting and not being coerced. This contract is legally binding and grants benefits and responsibilities to the citizens who enter into it. These incentives and responsibilities are coming from federal, state and local government.
The problem is that the government should have never gotten involved in marriage. They should have left that to religious organizations. The morals, customs, benefits and restrictions of those religions could apply. That isn't how it played out so now we are stuck with confusion.
I expect at some point that we will see a legal challenge on plural marriage. There should be no reason legally to prevent it. I am not aware of any other legal contracts that put restrictions on the number of adults who can enter into an agreement.
As for Davis, she was a State Official. Let's not forget that she was refusing to issue ANY marriage licenses. She should have been put in jail until she either complied with the duties of her office, resigned or was removed. Imagine if someone decided to refuse to issue hunting or fishing licenses because they thought killing animals was immoral. Or they refuse to give a driver's license or register a vehicle because they felt it was damaging the environment or facilitating drunk driving. As an elected official your morality is irrelevant. You were elected to carry out the laws of the state. If you are not willing to do so, then resign or go to jail. Pretty simple process.