IMHO, everything, everyday is a form of contract. We have a contract to drive on our side and we bet our life that everyone will honor that contract. We contract our labor so you don't find out at the end of the week what your pay rate was. In fact, we're all experienced contractors and the only place we don't use those skills is in affairs of the heart.
People get married for the dumbest reasons. You were good in bed so you'll also be good at life? We dive into a veritable swamp of legal and structural repercussions on vague assumptions. The only people that have any chance of having a fulfilling long term arrangement are people who know what the damn deal is. Polygamy would force people to give a lot more thought to how they entangle themselves.
A better and happier world filled with rainbows and unicorns.