User blog comment:B214/Question and Answers 3/@comment-62.178.18.58-20140813224147/@comment-62.178.18.58-20140814141744

Asia knows nothing of the underworld way of life as well as Ise at that point in time. And the parents of Ravel do not mention marriage in any form. The only thing they mention is that she becomes his servant. Raiser for example has a relationship with all of his servants expect his sister but none are married to him as far as we know.

 Take for example the dinner in the house of Gremory. We know that Rias father has a harem. But none of the woman were there for dinner. Only Rias mother and father.

 If one can marry more than one women than a lot of other problems arise as well. For example the succession of the family. Let’s say he had married 3 woman. How do you choose who will be next heir of the family. This only gives rise to intern power struggles and is bad for the family and so on.