Nowhere does the 2A require citizens to belong to the milita in order to exercise the right to keep and bear arms.  The point of an armed citizenry is so that when the militia is needed, they are ready and capable.  But if you really want to get into the weeds of it, the US militia code is quite clear.  It is also sexist and ageist, but clear nonetheless.  The Militia is the citizenry, period.