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?
Dude, it’s a contract. Read it before you sign it. If you don’t like it or don’t understand it, don’t sign it. It’s really not that complicated.
There isn't a current standard between the states. Some recognize SSM, some have laws against.
Usually contracts stipulate the state who's laws apply (usually the state of incorporation or primary headquarters).