In Game of Thrones we know that Jaime cannot marry, as Kingsguard members are supposed to not have family, as they took the oath to protect the king. I believe this oath is the same like the people at the Wall take.
On the other hand, we see Ser Loras Tyrell was supposed to get married to Sansa or Cersei, while he was Kingsguard for Renly and I believe he is Kingsguard for Joffrey and Tommen. Isn't he supposed to be bound with such an oath?