It's funny how that stupid piece of paper makes everything more "official" in other people's eyes.
When my husband and I got married, we stood in front of our families without an officiant and gave our vows to one another and it was emotional and sincere and important. Somewhere along the line our marriage license didn't get turned in, and it expired by the time we got back from the honeymoon.
We didn't particularly care, other than the minor inconveniences it caused, and we joked about it, saying we'd do it on our first anniversary, so at least half the date could be right.
When my father-in-law got wind of this, he cheerfully referred to me as "The False Wife" and badgered us for months about when we were going to get the paperwork done. We let him know repeatedly how unappreciated and unacceptable his comments were, but to no avail. Finally, my husband told him straight up that we weren't going to be seeing them anymore because of his rude and disrespectful behavior. And true to his word, we didn't see them for a very long time after that.
As it happens, we signed the papers on our second wedding anniversary, and when we asked if they would like to witness, they assumed we were having another wedding! Um hello, we already did that part!
Leaving the legal ramifications for another time, the moral of the story is that it's the intention of the act that matters, not the piece of paper, regardless of whether or not you're poly.