Today in 1893 U.S. President Benjamin Harrison declared full amnesty for Mormon polygamists. Is it the government's place to define which marriages are valid and which are not?
More basic is the question of "who gets to decide what the government's "place" is. If my voice counts in that, I want the government to be a registrar of contracts, and in that capacity it can only have a voice in what is being registered to the extent it should be legally enforced. And I don't think the government should have any voice in *who* gets married to whom, or why, or for what purposes. Who inherits or has responsibility for raising kids, sure.