I learnt that the UK has a much higher instance of mixed race marriages and children than in the US. At any rate, here I figured to offer my ideas on how each of the four cultures regard mixed race relationships or "miscegenation".
Gallente
Gallente Prime was likely globalized via peaceful means during their information age, and it is likely that all ethnic Gallenteans originating from Luminaire are all mixed race themselves, through the creation of a "world culture" and cross-cultural contact. With the "Immigrants" ancestry, it indicates that "ethnic Gallenteans" may also have immigrant blood to them (sort of like US Polish-Americans, German-Americans etc.)
However, you're definitely going to have the racial purity types of Luminaire, who are irritated how the ethnicity and culture of their homeword has been tainted by ze blood of a thousand immigrants or whatever. Traditionalist Jin-Mei and Intaki will not approve of marrying outside the culture perhaps due to Reborn/Caste reasons perhaps.
Minmatar
If Tribe is tied to ethnicity, then this is very likely frowned upon. A Tribe's identity would likely be threatened if they mixed about. However, since 1/5 of Minmatar are in the Feds (those varying between tribal traditionalists or being Gallenteanized), then they might have an occurence of being mixed. If they chose to return to the Republic, how would they be treated?
Caldari
For a nation that wishes to hold onto its cultural identity from external influences, then marrying outside the culture is probably disliked heavily. We also have distinct Civire and Deteis ethnicities; I began to wonder that since Civire are the workers and Deteis are the leaders, then there isn't much opportunity for Civire/Deteis to establish relationships or contacts with each other, as there would be a class/lifestyle gap between them.
Amarr
Frowned upon again here probably, if True Amarr are the "chosen race"? Don't want to taint the chosen race with the genes of a foreign bloodline of course. Likely instances of True Amarr holders smexing up slaves and having illegal offspring though.
Anyway, would like to hear your thoughts and counterthoughts on this.