Granted, marriage can be a convenience and more sociably acceptable.
But how many people pretend to be sociably acceptably married, and lie to the public, ~~while they live an opposite (sociably unacceptable) lifestyle and at the same time attempt to practice honesty with their spouse(s) and their polyamory friends?
Practicing honesty is a challenge. I think we are programed to "play the game" (with society) and it becomes a habit. I don't know if I could lead a double life (pretending to be in a sociably acceptable monogamous marriage) to some, while being honest and open to selected others. I would think developing trusting relationships might be tricky.