Why does somebody have to be married to have children? You don't have to be married to change a name, either. Although I don't think anyone in the Gedde family was ever named 'Laveau' - the game suggests that was more of a pseudo-name.

Having children out of wedlock is frowned upon. It seems odd that the family keeps finding men who are willing to do so.

Malia doesn't say that her family hasn't married - only that she doesn't choose to do it herself. It's quite possible that other Geddes in the past have married (and from Malia's comments we can assume those marriages were quite short!) or it's equally possible that they all chose not to. The Gedde family is clearly revered through New Orleans' society for their wealth and philanthropic efforts - perhaps their choices not to marry have been regarded as part of their eccentric mystique?

Tetelo's descendants have gone by the surname Gedde at least since they arrived in New Orleans, or maybe even before then.

Either the Gedde matriarchs never got married, or they kept their maiden name even after getting married. I would think that either possibility would raise a few eyebrows.

On the other hand, the Gedde family were extremely powerful, in more ways than one. Even those who would disapprove of the Gedde matriarch having a baby out of wedlock, or keeping her maiden name despite her marriage, wouldn't voice that disapproval too loudly.

As for finding a man willing to have children with the Gedde matriarch without marrying her... even if her beauty and wealth couldn't attract even one such man, Tetelo's magic certainly could.