The unnatural position argument is inconsistent. IMO the rule should be based on whether an arm is used to stop a cross or goal where there was no way the ball was being stopped if the arm wasn’t there, i.e. no other body part behind it.
Is that the rule nowadays? Seems well odd. I can understand a pen not given if it hits the arm in front of the body, but this stopped a cross coming in front of goal. Should be a pen really