Maybe someone has answered already. The married or not married has to do with, if 0bama was born not on US soil, he is still a US citizen (although not natural born due to his father being a foreigner) IF his mother and father were not married.
If they were married, and his mother was under age as noted due to the law at that time, and he was born on foreign soil, he would not have even been any kind of US citizen at birth.
His parents being married or unmarried only affects his citizenship, if born off of US soil, and if his mother had not spent 5 years in the US after the age of 14, which she had not. It does not change his non-NBC status.
Rephrasing; whether they were legally married (bigamy) or not, if and again IF senior is the father, then junior is also a dual citizen???