I am lucky in that I live in a fairly liberal state- Maryland... but right across the river, 20 miles from me, they have a state, Virginia, with a seriously conservative state government right now.

But how does a change in the law in New York State change this, except for people in New York State?

I'd have thought that changing the status of civil partnership right across the whole country so that it carried the same rights and responsibilities as marriage would be less of an uphill struggle than changing the definition of marriage so that it covered civil partnership as well. Doing it that way seems to have worked out pretty well in a wide range of countries.