Personally I don't really mind who does or doesn't get married unless it involves being a parent, because there is good evidence relating to that.

What surprises me is an argument that claims to be based on justice/fairness but whose demanded alternative simply widens the beneficiaries of the injustice. As you have noted, there is a very obvious and undeniably fair solution: simply do away with the legalized reference to marriage. Revert to common law/civil contract, which purportedly is all "marriage" really is according to many proponents for change. In that case the label can be preserved for people who value its traditional/cultural meaning and everybody else can push for whatever common law or civil measures they please.

Surely that's the obvious solution- unless there is value seen in forcing a change to the institution of marriage?