And when would this ever occur?
Children of foreign nationals inherit the foreign nationality of their parents, even if they are born citizens here, and even if they never choose to act on that.
If Rubio’s parents had become citizens and renounced their allegiance to Cuba before he was born, he would have been a natural born citizen.
Rubio’s children are natural born citizens because both of their parents were citizens.
Rubio is not a natural born citizen because his parents were still Cuban citizens and if they had gone back to Cuba, Rubio would be a Cuban citizen, too.
Rubio could have been a Cuban citizen if he had chosen to be.
Only when one can only be a US citizen and nothing else is one NATURALLY a US citizen. That only occurs when one is born here od citizen parents. Then there is no alternative, one naturally MUST be a US citizen.