They are not equivalent. I keep saying this but you keep ignoring it. There is zero connection between a domestic partnership and marriage. It simply allows for things like visitation rights which you may not get if you are not the single nominated partner. This simply allows people to nominate more than one person.
These are quite common around the world and which provides (from wiki) ‘an interpersonal status created by local municipal and county governments, which provides an extremely limited range of rights and responsibilities’.
The municipality of Massachusetts Town has simply allowed for more than one person to access these rights.
Before ssm was legal, some couples used this facility to ensure that their partner had, for example, visitation rights as did married couples. But it wasn’t - and isn’t, a precursor to changing what constitutes a marriage.