It doesn’t have to physically exist. A verbal contract is fine: “I love you forever.” “I will only be with you.” “I want to be exclusive with you.”
Discussion
You say they are consummating before marriage (which is illogical) and then claim they are consummating after marriage (which was my argument).
A marriage contract is generally sealed by sexual intercourse conducive to procreation, which makes it different from other contracts.
You can't accidentally get knocked up opening a bank account online or getting a car loan.
Im saying these things are arbitrary. Some people get married without the intention of procreating.