since some sort of legal status is required for many
purposes in most countries
The idea here is that you get rid of those "many purposes" and treat a married couple like any other people who have decided to spend their lives together.
that is what "married" means. Two people who have entered into a legal contract to spend their lives together and bear certain responsibilities with respect to each other and those in their custody. The religious interpretation of what it means is irrelevant for legal purposes in the US
