I think it can be hot for a couple to explore things like bringing in other partners, as a way to experiment, explore, and acknowledging natural attraction to others. This could be a good way to take trust to a whole new level.
But, I don't see it as something that is necessary nor something that would work well as a 24/7 thing...probably more like on rare occasion (happy birthday baby...
). Again, not needed at all, really, particularly if there is insane amounts of love in the relationship where both partners are totally satisfied by the other.
Otherwise, to each their own.
As for the government, I don't see the need for the government to recognize anything other than legal documents that can/should be drafted by any consenting adult. I don't see the need for government at any level to be directly involved in marriages.