The two abreast argument is specious in almost all cases, unless the road is very wide.
it's made by people in cars who want to (unsafely) squeeze past a cyclist who they think should be hugging the gutter. Even if the cyclist is on the white line, if the car gives the recommended (if not required) 3 or 4 feet, few roads in the country would allow that without the car crossing the center line.
Given that the car is going to cross the center line anyway, it can't pass until the way is clear. If the way is clear, then fully changing lanes is not a problem anyway.
So why on a narrow road is riding two abreast a problem?
I have noted that car drivers these days seem to have a serious aversion to turning the wheel more than a few degrees. I'm serious about this. I have noted that when pulling into or out of the subdivision I live in (I have the corner lot) people tend to go very wide, driving in the oncoming lane for a couple hundred feet or more, unless there's oncoming traffic there. And they like to cut the corner, driving on my lawn a lot (do I sound old?). When passing people want to just barely turn the wheel.
__________________
Work: the 8 hours that separates bike rides.