Thank you everyone who has replied so far. The help is really appreciated.
I think the friendship/children analogy is a fine idea. Any other on ways to explain things in a similar fashion are greatly appreciated.
I understand that my partner most likely may never change his position on monogamy, I suppose I feel like I owe it to us both to discuss it.
The difficult part then will be deciding between committing to the closed relationship and getting married or moving on.
This is awfully difficult isn't it?