Why is Monroe and Rosalee's relationship taboo when others aren't?

Throughout the series we see several couples that are not the same species of wesen, in Season 3 Episode 10 Alicia and Joe are different, Alicia is a Fuchsbau and Joe is a Klaustreich. Then the Season 1 Episode 16 we have a klaustreich married to a Seltenvogel. So, obviously there are cases when different wesen are allowed to be in a relationship with each other. Why then is Monroe and Rosalee’s relationship considered Taboo and a subject of such contention in the show when they seek to get married?