If it is about sharing money, then there are joint bank accounts.
If it is about inheritance, then there are wills and beneficiary declarations in insurance policies.
If it is about taxes, then is individuals filing.
If it is about mortgage deductions, then there is shared equity.
If it is about parenting orphans, then there is single parent adoptions.
But if it is about a visible sign of faith in an invisible reality formed by a lifelong union between a man and a woman, then there is marriage.
The government tells us to respect many things including Islam.
The government too should respect certain things e.g. the separation of Church and State and with that institution and sacrament of marriage.