It doesn't matter who invented it. It matters what it is. And marriage is fundamentally a government institution. It concerns laws and legally granted privileges. If there's a sacred component, let the churches keep that part, but that's not what the SCOTUS ruled on, and it's not what millions across the country have been demanding for decades.
Not legally you won't. Your "marriage" will be equivalent to "monogamous relationship", that's about it. No visitation rights. No tax breaks. No inheritance rights. No pension rights.
Currently marriage involves third parties treating couples differently because they're married. That requires a standardized status and seems tricky to do with contract.
For example: different taxation, different financial aid, etc.
You could get rid of all these, but that is a big change.
