I couldn't find a thread about this matter when I searched (and I searched quite long ).
There is a thread about this subject on Skadi, so I thought the question could be brought up here too.
If you vote the third option, please elaborate. I would choose that option, and thus will do.
As the alternative says, only if the two types are somewhat close to each other and falls into the "indigenous" category of a country.
My family is quite "mixed" itself, with my father being very Nordid with some West Baltid influences (thus making him a "homemade Trųnder" type). I would think that my paternal grandmother is a North-Atlantid (very Nordid with dark hair) and that my maternal grandfather is a textbook Dalo-Nordid, more or less. My Baltid phenotype comes from my maternal grandmother. I don't know my paternal grandfathers type since I've never met him and there are only bad pictures of him, but judging from those pictures, he seemed to have been at least mesocephalic. My younger brother reminds much of our father, though he has green eyes (father has blue). Other than that, I have uncles and aunts falling into the Borreby category.
This of course makes me "partial". My family falls within the Northern (both North-Western and North-Eastern) category, so I'd have to say that I wouldn't be against a marriage between a Faelid / Brünn and a Nordid for instance. Neither I wouldn't be against a Baltid / West Baltid marrying a Borreby or Nordid.
But when it would come to cases like Mediterranid + Borreby or Dinarid + Nordid... Well, at least I could say that I wouldn't recommend it.