On fenoxo forums there was a statement that PC marriage only planned for nobles and royals.
Being able to marry Brint means he's a royal too.
Pretty sure it was "mainly/mostly", not "only". I'd be very surprised if by the end of the game you can't marry most, if not all of the companions.
So far, we can marry Kiyoko, Tetsuya, Ahmri, Atani.
Upcoming are Brienne/Brint, Cassia, Rina, Carmen, Etheryn, and Kythie.
So the three moos are the odd ones out at the moment, as they are the only ones not confirmed as any kind of nobility...yet.