A contract is a contract. It wouldn’t matter what other people “saw”. If they want to sue each other over it, that’s fine with me. Stupid people should be separated from their money, even if that means getting lawyers involved.
Many car rental companies as a matter of policy will allow a renter's spouse to drive a rented car without having to pay an "additional driver" fee. If there isn't a standard definition of "marriage", how should the company determine whether or not two people who wish to rent a car should have to pay the extra fee? The question "Are these people married" should be a simple one, but if every "marriage" contract is different how should the company determine whether two people are actually married or just drew up a sham contract to save a few dollars on their car rental?