A contract is a legal term. It can be enforced by the laws of the land, and you can use the courts to enforce it.
I believe religion is personal to an individual. I do not believe in any group religion. When one tries to impose their will on others then only conflict starts.
For all contracts and interpersonal relations, the laws of the land should be determining factor. Religion should not enter social, cultural, or legal matters.
Marriage or any other relationship should be dictated by the laws of the land.
let us say I enter into a business partnership, Religion has no saying in that. Similarly, marriage or cohabitation (without marriage) is governed by the laws of the land. Religion should not have a say in the laws governing marriage. The government of India is slow to enact a uniform law for all of India. They have allowed Muslim women some reprieve in the matters of Talak and divorce.
A bill criminalizing talaq-e-biddat, or triple talaq, was passed in India on July 30, 2019. The bill was generally regarded as a victory for Muslim women in India.
You can have a religious ceremony for any purpose. I did a grahpravesham ceremony for my house in the USA. It has no legal ramifications, but it has social significance for me.
You always talk about Islam as if it matters to all. To me, it has no meaning at all (i respect it for the people involved) just as I wear a head scarf when I go to Gurudwara.
It is pity that India does not have a uniform personal law. In the USA bigamy is ill-legal for all humans. That is the law of the land.