I just remembered that a man can choose to take on his wifes family name upon marriage... this has been known to happen in the case of famous and powerful families, or even to meet inheritance requirements. Its not unknown of.
Or in the cases where the man has been brow beaten and stripped of all his pride and manhood.
Since his grandmother (from Generations) had already established a career in Starfleet prior to being married, she probably didn't change her name upon marriage. Then all she had to do was have a son with the surname "Sulu."
This doesn't explain why Akira Sulu is a Native American and not an Asian, though...