Yes, the answer is there is no reason to get divorced if you aren't married. No one would do it. Hence we can infer from their divorce that they did go through with a marriage ceremony. Perhaps their that marriage wasn't legal, but it did happen and is presumed legal and valid until anulled.
Bigamy is illegal in all 50 states of this country. Stanley Ann Dunham could not have married Barack Hussein Obama in the United States because he already had a wife in Kenya. If they were married in Hawaii because he and possibly Stanley Ann lied, then that marriage would be null and void and a divorce would have no legal standing,
I suppose it is possible that Obama Sr. committed bigamy when he married SAD. Why should I care?