“Can you be married but not legally married?
No. Marriage is a legal arrangement under civil law. That means marriage in the United States is restricted and governed by state laws. Therefore, in order to be considered legally married the parties must meet all the requirements for a valid marriage in the state where they will be married.”
http://www.answers.com/Q/Can_you_be_married_but_not_legally_married
Yes but what it says is that two people can go through a marriage ceremony, consider themselves married (or at least one of them may think so) but not be legally so - if one of them is already married for example.
In this case, that is all moot. In retrospect, it might be possible to argue that given Obama Sr’s previous marriage in Kenya, his marriage to Dunham in Hawaii wasn’t legal. However, the contemporary divorce documentation show’s that the court had proof presented to it that Obama Sr and Dunham were married in February 1961 in Hawaii and that the divorce was granted for grievous mental suffering with no mention of putative status or bigamy.
Bar a copy of a the original marriage certificate being found, that is about as definitive as anyone is going to get that there was a marriage ceremony on that date in Hawaii - which was during Dunham’s pregnancy.